Effect of the temperature on the microstructure and the mechanical properties of laser cladded Ni-based metal matrix composite coatings

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MICROSTRUCTURE AND THE

MECHANICAL PROPERTIES OF LASER

CLADDED Ni-BASED METAL MATRIX

COMPOSITE COATINGS

Davide Verdi

PhD THESIS

Móstoles (Madrid), Spain, 2015

THESIS DIRECTORS: Dr. Pedro Alberto Poza Gómez

Dr. Claudio José Múnez Alba

Departamento de Tecnología Química y Energética, Tecnología Química y Ambiental,

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Don Pedro Alberto Poza Gómez, Catedrático de la Universidad Rey Juan Carlos.

Don Claudio José Múnez Alba, Contratado Doctor de la Universidad Rey Juan Carlos.

CERTIFICAN

Que el presente trabajo de investigación con el título “EFFECT OF THE TEMPERATURE ON THE MICROSTRUCTURE AND THE MECHAN-ICAL PROPERTIES OF LASER CLADDED Ni-BASED METAL MATRIX COMPOSITE COATINGS” que construye la Memoria presentada por Davide Verdi para optar al grado de Doctor de la Universidad Rey Juan Carlos con mención “Doctor Europeus”, ha sido realizada bajo su supervisión en el Departamento de Tecnología Química y Energética, Tecnología Química y Ambiental, Tecnología Mecánica y Química Analítica de la Universidad Rey Juan Carlos, cumpliendo todos los requisitos necesarios.

Y para que así conste a los efectos oportunos lo firman en Móstoles,

a de de .

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T

his work is the result of years of research that would not be possiblewithout the support, advice, and guidance of several persons. Every-one collaborated directly or indirectly on this project and I will thank you all.

My acknowledgements are firstly direct to my thesis directors Dr. Pedro Poza and Dr. Claudio J. Múnez, to believe in me and give me the oppor-tunity to develop this research. You two and all the other members of the

DIMME (Durabilidad e Integridad Mecánica de Materiales Estructurales) group of the Universidad Rey Juan Carlos gave me the chance of work in a professional and at the same time comfortable and friendly environment. Thank you for all the lessons and the diversion.

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Another particular acknowledge to “mi colega” David for all the support in the laboratory tasks. Thank you not only for your work, but especially for the happy times together.

I will also desire good luck to Alberto, Marcos, Marlen, and Paloma for their PhD.

Thank you also to Jesús González Casablanca for his lessons about the samples preparation and the help with the TEM.

I would also to acknowledge Dr. Chris Allen for his supervising work and all the support that he gave (and still give) me during my predoctoral stay in TWI Ltd (Cambridge, UK). Thank you to all the members of the LAS department and to Dr. Phill McNutt and Dr. Tiziana Marrocco for their help during my stay at the TWI Technology Centre - Yorkshire (Catcliffe, UK).

I follow acknowledge Bibiana, Lus, Manu, and Marcelo for the good times passing together in the University, talking about everything and demonstrating that the work have not to affect the personal life.

It is not possible to forget you Fran. You surely transmit me part of your enthusiasm for the investigation, and sometimes I found myself think about how this work had been with you.

Thank you to all those shared the house in Madrid with me in this period, especially to Hugo, my ideal flatmate.

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Thank you to my friends Sergio and Victor, not only for our mad nights in Madrid, but also for all the time and the words you dedicate to me. I want also to acknowledge my old friends Enzo, Fulvio, Gio, and Marzio: though time moves on and we see one each other just occasionally, I know I will always count on you.

It is impossible do not mention my family, my mother Anna and my father Giuseppe, my brother Cristian and my sister-in-law Ramona, and especially those two lovely earthquakes that are my little nieces Nicole and Letizia. Thank you for the smiles, the hugs, the discussions, the Skype conversations, etc. You are a constant support in my life!

Last but not least, I want to thank you Daria. Since I met you (well . . .

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Sin embargo, la fragilidad de estos materiales puede llegar a ser una dificul-tad al utilizarlos industrialmente como recubrimientos, tanto durante su vida en servicio, como a la hora de depositarlos sobre los sustratos. El carburo de cromo, por ejemplo, se emplea como recubrimiento para proteger materiales de menor resistencia al desgaste por deslizamiento [2]. Este cerámico es uno de los más empleados en forma de recubrimiento en la industria química y en aplicaciones estructurales, en rodamientos y juntas. Existen varios trabajos sobre las ventajas de utilizar recubrimientos de carburo de cromo para mejorar la resistencia al desgaste [3–5].

Los Cermets y los materiales Compuestos de Matriz Metálica (Metal Matrix Composite, MMC) son soluciones innovadoras que combinan las propiedades de los metales y de los cerámicos para mejorar las característi-cas y la vida en servicio de componentes, por ejemplo hacia temperaturas de trabajo más altas o cargas más severas. En estos materiales, el metal trabaja como aglutinante para las partículas cerámicas, reduciendo de esta manera la fragilidad del componente final. Se pueden emplear diferentes metales en función del tipo de aplicación y de refuerzo. El carburo de cromo se suele combinar con aleaciones de base Ni-Cr para aplicaciones a altas temperaturas [6].

Las técnicas de proyección térmica como la proyección a alta velocidad (High-Velocity Oxy-Fuel Spraying,HVOF) y la proyección por plasma

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Como menciona J.C. Betts [9], es posible encontrar diferentes trabajos de investigación sobre mezclas de carburo de cromo y metales obtenidos por deposición láser. En la mayoría de estos la deposición del recubrimiento se hizo por medio de un láser de CO2 [8, 10] o Nd:YAG [6]. Además, en su artículo, Betts proporciona un listado de trabajos en los que diferentes autores estudian las propiedades tribológicas de cermets donde el carburo de cromo constituye la fase de refuerzo. En todas estas referencias las pruebas fueron efectuadas en una escala macro con ensayos de erosión [11] o de tipo punzón sobre disco (pin-on-disc) [8,10,12], también a altas temperaturas [13].

El trabajo realizado en esta tesis permite evaluar el efecto de introducir partículas de carburo de cromo de estequiometría Cr3C2 en una matriz metálica constituida por una superaleación de base Ni. Se han analizando las variaciones de microestructura, propiedades mecánicas, y de comportamien-to local frente al desgaste. También se analizaron los efeccomportamien-tos de la exposición a alta temperatura. El estudio se llevó a cabo sobre la matriz de un recubri-miento de MMC constituido por Inconel 625 y Cr3C2 (IN625−Cr3C2), y se utilizó un recubrimiento de Inconel 625 (IN625) como referencia. Ambos sistemas se depositaron sobre un sustrato de acero medianteLC.

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El trabajo presentado es novedoso con respecto a otros como los citados por Betts [9] en varios puntos. En primer lugar por el tipo de láser usado, que en esta investigación fue de diodo de alta potencia (High-Power Diode Laser,

HPDL). Además, el estudio tribológico se realizó en escala microscópica y no se han encontrado trabajos sobre el comportamiento frente al desgaste en esta escala sobre recubrimientos deIN625y materiales compuestos de matrizIN625. Finalmente, la influencia de la temperatura en la respuesta al desgaste y las propiedades mecánicas de estos recubrimientos es aún desconocido.

Por estas razones, los objetivos principales de esta investigación son:

1. Seleccionar una combinación de parámetros del sistema de proyección por LC adecuado para generar recubrimientos de IN625−Cr3C2 e IN625sobre un sustrato de acero.

2. Evaluar el efecto de la incorporación de partículas de Cr3C2 sobre el IN625en términos de:

• microestructura;

• propiedades mecánicas;

• comportamiento local frente al desgaste.

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4. Establecer una correlación entre las variaciones de microestructura, propiedades mecánicas, y comportamiento local frente al desgaste que tenga en cuenta la incorporación de partículas de Cr3C2 y la exposición a altas temperaturas.

Como se ha mencionado anteriormente, los recubrimientos estudiados se depositaron por plaqueado láser. Previamente, se estudió el proceso de fabricación para seleccionar una combinación de parámetros de deposición adecuada. Potencia del haz láser (P), cantidad de material de aporte (Q), y velocidad de barrido (v) fueron los tres parámetros optimizados. El recubri-miento deIN625se obtuvo proyectando un polvo comercial. En el caso del recubrimiento deMMC, el mismo polvo de IN625se mezcló mecánicamente antes de su proyección con un 20 % en peso de polvo comercial deCr3C2. Las deposiciones se realizaron sobre dos tipos de sustrato previamente gra-nallados: acero ferrítico de Gr22 (ASTM A387 [16]) y acero inoxidable 316L (ASTM A276 [17]).

Una vez obtenidos los recubrimientos, se sometieron a oxidación isoterma en aire. Las muestras se introdujeron en un horno de laboratorio a 520 y 800 oC durante diferentes tiempos de exposición: 24, 48, 72, 168 y 336 horas.

Las muestras se prepararon para su caracterización microestructural an-tes y después de oxidar. Se analizaron por mediante microscopia electrónica de barrido (Scanning Electron Microscopy, SEM), de trasmisión (Trans-mission Electron Microscopy, TEM), ambos combinados con microanálisis de rayos X (Energy-Dispersive X-ray, EDX), y por difracción de rayos X (X-Ray Diffraction, XRD).

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usando un indentador tipo Berkovich, y siguiendo la metodología de medida continua de la rigidez (Continuous Stiffness Measurement,CSM) [18, 19]. En esta técnica se controla el desplazamiento del indentador dentro de la muestra y permite obtener un registro continuo de la evolución del modulo elástico (E) con la profundidad de penetración. En función de la carga alcan-zada a la máxima profundidad de penetración, fijada a 1500nm, es posible obtener valores de dureza (H) del material analizado. Para obtener un mejor promedio de las propiedades de los materiales estudiados, se programaron para cada muestra un total de 100 ensayos divididos en 5 matrices de 5×4 indentaciones en 5 zonas diferentes de la superficie de las muestras. Para completar el estudio, se realizaron ensayos de indentación Vickers (HV) a lo largo del perfil transversal de los recubrimientos, desde la superficie hasta una zona del sustrato suficientemente distante de la intercara. Este mismo tipo de ensayos de dureza se llevaron a cabo sobre la superficie pulida de las muestras y comparados con los valores de dureza obtenidos de las indentaciones instrumentadas.

Finalmente, el comportamiento local frente al desgaste se estudió me-diante pruebas de micro-rayado, realizadas también con un indentador tipo Berkovich. Para cada ensayo se calculó la tasa de desgaste local (k) divi-diendo el volumen desplazado por el indentador (Vd) por el producto entre la carga normal aplicada (FN) y la longitud de rayado (L). Se realizaron

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fijó en 300 µmy por cada muestra se programaron 15 ensayos repartidos en las mismas 5 zonas donde se efectuaron las medidas de Ey H. Los valores deVd se calcularon, mediante un modelo detallado en la Sección 3.6.1, a partir de las medidas de variación de profundidad y del semiángulo del perfil transversal del surco residual dejado por el indentador. La hipótesis principal en la que se fundamenta este modelo, es que el semiángulo del surco residual se mantiene constante a lo largo de toda la longitud de ensa-yo, incluso después de la recuperación elástica del material. La validez de este modelo se corroboró con medidas del volumen desplazado obtenidas mediante microscopia de fuerzas atómicas (Atomic Force Microscope,AFM). Pueden considerarse cuatro micromecanismos responsables del desgaste: deformación plástica, corte, fractura y fatiga [20]. De acuerdo con estas posibilidades se analizó el micromecanismo responsable del desgaste de los diferentes materiales estudiados, en las diferentes condiciones de oxidación.

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Las medidas efectuadas medianteAFMvalidaron el modelo propuesto para determinar la tasa de desgaste local en ambos materiales estudiados. Además, el modelo resulta válido incluso cuando un ensayo de rayado atra-viesa diferentes fases y la medida del semiángulo del perfil transversal se realiza en solo una de ellas.

La microestructura de los recubrimientos de IN625fabricados resultó homogénea y de tipo dendrítica. Se observó la presencia de segundas fases enriquecidas principalmente en Nb y Mo, identificados medianteTEMcomo aglomerados de carburosM2C y M6C, en las zonas interdendríticas. Los recubrimientos deIN625−Cr3C2, además de partículas de Cr3C2 no fundi-das o parcialmente fundifundi-das, presentaron una matriz deIN625modificada compuestas por zonas de diferentes tipologías. En la primera, cerca de las partículas primarias no completamente fundidas, fue posible identificar carburos de estequiometría M7C3 generadosin-situ y enriquecidos en Cr, Nb, y Mo. En la segunda, localizada en zonas más alejadas de las partículas no fundidas, se observó una fina estructura dendrítica en forma de hojas de helecho enriquecidas en Cr. No se encontraron diferencias significativas entre los plaqueados de IN625, como para los Metal Matrix Composite (MMC), aplicándolos sobre acero Gr22 o 316L. Los ensayos deDSIpermitieron con-firmar que también desde el punto de vista de las propiedades mecánicas, no hay diferencia entre depositar sobre acero Gr22 o 316L. Las modificaciones microestructurales debidas a la adición deCr3C2 conllevan un incremento del modulo elástico del 15-16 % y a duplicar la dureza delIN625.

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mayor o menor proporción entre ellas condiciona el comportamiento al desgaste del recubrimiento. En todos los casos, el principal mecanismo de desgaste resultó ser el micro-corte y la resistencia al desgaste mejoró de aproximadamente un 20 % con respecto al IN625 sin refuerzo cerámico. Calculando una tasa de desgaste total para los compuestos (midiendo una resistencia promedio de las partículas no fundidas y teniendo en cuenta su porcentaje en las diferentes muestras) se obtuvieron valores parecidos para los recubrimientos aplicados sobre acero Gr22 y 316L, destacando que el comportamiento global frente al desgaste de un material multi-fase puede diferenciarse del comportamiento local de cada fase por separado.

El proceso de oxidación a 520oC promovió la formación de nuevos car-buros y fases de Laves en las zonas interdendríticas de los recubrimientos de

IN625a partir de las 48horasde exposición. El mismo tratamiento no alteró cualitativamente la microestructura de los recubrimientos deIN625−Cr3C2 en todos los tiempos de exposición estudiados. Estas variaciones en la mi-croestructura se corresponden con un incremento deE yH observado en los recubrimientos de IN625. Por otra parte, estas propiedades mecánicas se mantuvieron prácticamente constantes en los MMC.

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La oxidación a 800oC modificó la microestructura de los recubrimientos de IN625 a partir de las primeras horas de exposición. La formación de precipitados en forma de agujas y otros de forma irregular se observó inicial-mente en las zonas interdendríticas del recubrimiento. Las primeras fueron identificadas medianteTEM como faseδ mientras que las segundas como fases de Laves. Al aumentar el tiempo de exposición a alta temperatura, el número y las dimensiones de estas fases crecieron hasta las 336 horas

cuando se encontró su presencia también en el interior de las dendritas. Este proceso de oxidación afectó también a la microestructura de la matriz del recubrimientoMMC. En la intercara entre la matriz deIN625y los carburos

M7C3 que se formaron durante el proceso de deposición láser, se identifica-ron carburos de diferente estequiometría (M C yM23C6) y especialmente enriquecidos en Nb. También se detectó la presencia de fases de Laves.

Las fases δ y de Laves son fases más duras y rígidas que la estructura

γN ique caracteriza el IN625. Como consecuencia, después de 336horas

de exposición, se observó un incremento de modulo elástico del 17 % y dureza del 48 %. En el caso de la matriz del recubrimientoIN625−Cr3C2, el aumento de EyH fue del 20 %.

Las tasas de desgaste locales estudiadas para las diferentes muestras de

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IN625−Cr3C2 apenas se modificó con el tratamiento térmico a 800oC. El corte fue siempre el mecanismo principal, mientras que el de micro-deformación plástica permaneció constante para los diferentes tiempos de oxidación. En todos casos, la resistencia al desgaste resultó superior a la del recubrimiento deIN625.

Bibliography

[1] D. Zhang and X. Zhang. Laser cladding of stainless steel with Ni-Cr3C2 and Ni-WC for improving erosive-corrosive wear performance. Surface

&Coatings Technology, 190:212–217, 2005.

[2] P. Hugh. Handbook of refractory carbides and nitrides. William Andrew Applied Science Publishers, New Jersey, USA, 1996.

[3] S. Söderberg, M. Sjöstraqnd, and B. Ljungberg. Advances in coating technology for metal cutting tools. Metal Powder Report, 56 (4):24–40, 2001.

[4] R. Wei, J.J. Vaja, J.N. Matossian, and M.N. Gardas. Aspects of plasma enhanced magnetron-sputtered deposition of hard coatings on cutting tools. Surface & Coatings Technology, 158-159:465–472, 2002.

[5] P. Willich and C. Steinberg. SIMS depth profile analysis of wear resistant coatings on cutting tools and technical component. Applied Surface Science, 179:263–268, 2001.

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[7] Z. Liu, J. Cabrero, S. Niang, and Z.Y. Al-Taha. Improving corrosion and wear performance of HVOF-sprayed Inconel 625 and WC-Inconel 625 coatings by high power diode laser treatments. Surface & Coatings Technology, 201:7149–7158, 2007.

[8] L. Wang, J. Zhou, Y. Yu, C. Guo, and J. Chen. Effect of powders refinement on the tribological behavior of Ni-based composite coatings by laser cladding. Applied Surface Science, 258:6697–6704, 2012.

[9] J.C. Betts. The direct laser deposition of AISI316 stainless steel and Cr3C2 powder. Journal of Materials Processing Technology, 209:5229– 5238, 2009.

[10] G.L. Goswami, S. Kumar, R. Galun, and B.L. Mordike. Laser cladding of Nickel based carbide dispersion alloys for hardfacing applications.

Lasers Engineering, 13:35–44, 2003.

[11] J. Nurminen, J. Näkki, and P. Vuoristo. Microstructure and properties of hard and wear resistant MMC coatings deposited by laser cladding.

International Journal of Refractory Metals& Hard Materials, 27:472– 478, 2009.

[12] H. Luo, K. Gong, S. i, X. Cao, X. Zhang, D. Feng, and C. Li. Abras-ive wear comparison of Cr3C2/Ni3Al composite and Stellite 12 alloy.

Proceeding of Sino-Swedish Structural Materials Symposium, 2007.

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[14] J.C. Ion. Laser Processing of Engineering Materials. Principles, pro-cedure and industrial application. Elsevier Butterworth-Heinemann,

Oxford, UK, 2005.

[15] R. Vilar. Laser Powder Deposition, in Comprehensive Materials Pro-cessing, Volume 10. Elsevier, Amsterdam, Netherlands, 2014.

[16] ASTM A387/A387M-11. Standard Specification for Pressure Vessel Plates, Alloy Steel, Chromium-Molybdenum. American Society for Testing and Materials, 2011.

[17] ASTM A276-13a. Standard Specification for Stainless Steel Bars and Shapes. American Society for Testing and Materials, 2013.

[18] X. Li and B. Bhushan. A review of nanoindentation continuous stiffness measurement technique and its applications.Materials Characterization, 48:11–36, 2002.

[19] W.C. Oliver and G.M. Pharr. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. Journal of Materials Research, 7:1564–1583, 1992.

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Page

Aknowledgments VII

Resumen XI

Bibliography . . . XXI Acronysm and symbols XXXI

1 Objectives 1

Bibliography . . . 5

2 Introduction 7

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2.2 Metal Matrix Composite . . . 15

2.3 Surface engineering . . . 18 2.4 Laser surface modification . . . 26

2.4.1 Laser cladding . . . 36 Bibliography . . . 40

3 Materials & Experimental Procedure 47 3.1 Materials . . . 48

3.1.1 Filler powders . . . 49 3.1.2 Substrate materials . . . 56 3.2 Laser Cladding process parameters selection and

coatings production . . . 60 3.3 High temperature isothermal oxidation tests . . . 66

3.4 Microstructural characterization . . . 67 3.5 Mechanical characterization . . . 69

3.5.1 Depth Sensing Indentation tests . . . 70 3.5.2 Vickers microhardness . . . 83

3.6 Local wear behaviour . . . 85 3.6.1 Model used for the calculation of the local

wear rate . . . 87 3.6.2 Local scratch tests procedure . . . 92 Bibliography . . . 93

4 Laser Cladding coatings process 101 4.1 Single track tests . . . 102

4.1.1 Inconel 625 single track tests . . . 103 4.1.2 Metal Matrix Composite single track tests 111

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Bibliography . . . 120

5 Microstructural characterization 125 5.1 As deposited Inconel 625 clads’ microstructure . . 127

5.1.1 Inconel 625 deposited onto Gr22 ferritic

steel . . . 127 5.1.2 Inconel 625 deposited onto 316L stainless

steel . . . 131 5.1.3 TEManalysis of the as deposited Inconel 625

clads . . . 135 5.2 As deposited Metal Matrix Composite clads’

microstructure . . . 141

5.2.1 Metal Matrix Composite deposited onto Gr22 ferritic steel . . . 143 5.2.2 Metal Matrix Composite deposited onto 316L

stainless steel . . . 150

5.2.3 TEManalysis of the as deposited MMCclads’ matrix . . . 160 5.3 Inconel 625 clads’ microstructure after isothermal

oxidation tests. . . 165 5.3.1 Oxidation tests at 520 oC . . . 165 5.3.2 Oxidation tests at 800 oC . . . 177 5.4 Metal Matrix Composite clads’ matrix

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6 Mechanical properties 221

6.1 Depth Sensing Indentation tests results . . . 222 6.1.1 Inconel 625 clads . . . 229

6.1.2 Metal Matrix Composite clads . . . 234 6.2 Vickers microhardness tests results . . . 247

6.2.1 Cross section tests. . . 247 6.2.2 Plain view tests . . . 253 Bibliography . . . 257 7 Local wear behaviour 261

7.1 Validation of the model used to calculate the local wear rate . . . 262 7.2 Preliminary local scratch tests on different

deposited laser beads . . . 268 7.3 Parameters selection for the local scratch tests 269 7.4 Local scratch tests results . . . 274 7.4.1 Inconel 625 clads . . . 274 7.4.2 Metal Matrix Composite clads . . . 277

Bibliography . . . 286

8 Discussion 289

Bibliography . . . 316

9 Conclusions 319

10 Future works 325 A Instrumented indentation models 327

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A.2 ISE analysis models . . . 328

A.2.1 Nix-Gao model . . . 328 A.2.2 Meyer’s law . . . 329

A.2.3 Hays-Kendall approach . . . 329 A.2.4 Elastic recovery model . . . 329 A.2.5 Proportional Specimen Resistance model . . 330 A.2.6 Modified Proportional Specimen Resistance

model. . . 331

Bibliography . . . 331 B Statistical analysis of the data 333

B.1 Introduction to the error analysis . . . 333 B.2 Probability distribution . . . 335 B.2.1 Normal distribution . . . 335

B.2.2 Rectangular distribution . . . 336 B.2.3 Triangular distribution . . . 337

B.2.4 U-quadratic distribution . . . 337 Bibliography . . . 337 C Depth Sensing Indentation tests data: bin histograms 339 C.1 Inconel 625 clads oxidised at 520oC . . . 340 C.2 Inconel 625 clads oxidised at 800oC . . . 342

C.3 Metal Matrix Composite clads oxidised at 520oC. 344 C.3.1 Inconel 625−Cr3C2 clads’ matrix oxidised at

520 oC . . . 348 C.3.2 Unmelted Cr3C2 particles of the MMC clads

oxidised at 520 oC. . . 350

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C.4.1 Inconel 625−Cr3C2 clads’ matrix oxidised at

800 oC . . . 356 C.4.2 Unmelted Cr3C2 particles of theMMC clads

oxidised at 800 oC. . . 358

D Numerical values 361

List of figures 367

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A cross section of the scratch residual groove

A1 clad area

A2 molten area

Ac contact area

Ac,real real contact area

AFM Atomic Force Microscope

APS Atmospheric Plasma Spraying

AP U area of the piled-up material

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BCC Body-Centred Cubic

BCT Body-Centred Tetragonal

BF bright field

BSE Backscattered Electrons

CAPS Controlled-Atmosphere Plasma Spraying

CDF centred dark field

CGDS Cold Gas-Dynamic Spray

CI confidence interval

CS Cold Spray

CSM Continuous Stiffness Measurement

CVD chemical vapour deposition

D distance between the centres of neighbouring laser tracks

d average dimension of the unmeltedCr3C2 particles

D% dilution

D-GUN Detonation-Gun Spraying

DOE Design Of Experiment

DSI Depth Sensing Indentation

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E∗ reduced elastic modulus

Ec elastic modulus of “cutting”

Em elastic modulus of the matrix

Ep elastic modulus of the unmeltedCr3C2 particles Esp elastic modulus of the secondary phases

(E/H0)norm normalizedE/H0 ratio

EDX Energy-Dispersive X-ray

ESW electroslag welding

F indentation load

fab Zum Gahr parameter

Fmax maximum indentation load

FN normal load

FCC Face-Centred Cubic

FS Flame Spraying

GMAW Gas Metal Arc Welding

GNDs geometrically necessary dislocations

GTAW Gas Tungsten Arc Welding

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hc contact depth

hc,real real contact depth

hr residual impression depth

H hardness

H0 asymptotic hardness

H0,c asymptotic hardness of “cutting”

H0,m asymptotic hardness of the matrix

H0,p asymptotic hardness of the unmeltedCr3C2 particles H0,sp asymptotic hardness of the secondary phases

HAZ Heat Affected Zone

HCP Hexagonal Close Packed

HPDL High-Power Diode Laser

HREM high-resolution electron microscopy

HV Vickers microhardness

HVOF High-Velocity Oxy-Fuel Spraying

IN625 Inconel 625

ISE Indentation Size Effect

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k wear rate

kCRACK wear rate of microcracking

kCU T wear rate of microcutting

kF AT IGU E wear rate of microfatigue

km wear rate of theMMCclads’ matrix

km,CU T wear rate of microcutting of theMMC clads’ matrix

km,P D wear rate of microploughing of theMMC clads’ matrix

knorm normalised local wear rate

kp wear rate of the unmeltedCr3C2 particles kP D wear rate of microploughing

kt total local wear rate of the MMCclads

L scratch length

L1 height of the track

L2 depth of the substrate melted zone

L3 width of the track

LC Laser Cladding

LED Light Emitting Diode

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MMC Metal Matrix Composite

n number of observations

NBDP nano-beam diffraction pattern

OA oxyacetylene welding

OR overlapping ratio

P laser beam power

PDAS Primary Dendrite Arm Spacing

PMZ Partially-Melted Zone

PS Plasma Spray

PSR Proportional Specimen Resistance

PVD physical vapour deposition

PW powder welding

Q powder feed rate

R wear resistance

Rm wear resistance of theMMCclads’ matrix

Rp wear resistance of the unmeltedCr3C2 particles Rt wear resistance of multiphase materials

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SADP selected-area diffraction pattern

SD standard deviation

SDM standard deviation of the mean

SDAS Secondary Dendrite Arm Spacing

SE Secondary Electrons

SEM Scanning Electron Microscopy

SPGR space group

TCP Topologically Close-Packed

TEM Transmission Electron Microscopy

TS Thermal Spray

T-T-T time-temperature-transformation

v transverse scanning speed

vs scratch velocity

Vd displaced volume

VPS Vacuum Plasma Spraying

x scratch distance

XRD X-Ray Diffraction

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z0(x) initial profile

Z(x) displacement into surface at maximum load

z(x) residual profile

α semi-angle of the residual groove

λ indenter shape factor

ν Poisson’s ratio

σ standard error

σM standard error of the mean

Φ percentage of secondary phases

φ diameter of the equivalent circular contact area

ϕ percentage in volume of unmelted Cr3C2 particles

ψ plasticity index

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N

i-base superalloys are usually adopted in applications where the cor-rosion and the oxidation resistance is critical for the correct service of a mechanical part, i.e., in drilling and oil extraction, mining, thermoelectric plants, steam turbine, aircraft engines, etc. However, these alloys have a low resistance to wear and, in all the previously listed applications, static and dynamic parts are subjected to erosive processes, for example due to hard particles, like oxides, coming from the outside or directly generated in the system, that could hit and scratch the different parts promoting undesirable failures [1].

It is well known that ceramic coatings are the best solution to increase the working temperature of these components, improving their wear and corrosion/oxidation resistance. The main problem, regarding their ap-plication, relies on their brittle behaviour that could be problematic, i.e. during their treatment to generate a coating layer, for their reparations, or during the service life in case of blows. Chromium carbide is used as a coating to protect poorest underlying metals and also where sliding abrasion is present [2]. It is one of the most employed ceramic used as coating in applications like chemical equipments and structural parts like bearings and seals. It is possible to find different studies in literature about the advantages of use chromium carbide to improve the wear resist-ance [3–5].

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be selected. For chromium carbide, the Ni-Cr alloys are the most employed for applications at high temperatures [6].

While thermal spray techniques, in particular High-Velocity Oxy-Fuel Spraying (HVOF) and Plasma Spray (PS), are the most common methods to process composite coatings, with the appearance of high-power lasers, the Laser Cladding (LC) technique is a very efficient alternative in tech-nical and also economical terms [1, 7, 8]. The advantages of this technique compared with the thermal spray methods are described in Chapter 2.4.1. As mentioned by J.C. Betts [9], it is possible to find different research works where the microstructure of chromium carbide/metal blends were formed by laser deposition. Commonly, the deposition process was carried out with CO2 [8, 10] or Nd:YAG lasers [6]. In addition, Betts [9] supply a list of papers where different authors studied the tribological properties of cermet systems where the chromium carbide constitute the ceramic rein-forcement. In all these references, the tests were performed at macro-scale with erosion tests [11] and pin-on-disc tests [8, 10, 12], also at high temper-ature [13].

The research work performed and described in this thesis, evaluates the effects to introduce Cr3C2 chromium carbide ceramic particles in a Ni-base superalloy matrix, analysing the variation of the microstructure, mechanical properties and local wear behaviour. Two different systems were studied: an Inconel 625 (IN625) alloy coating, and the matrix of a

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As described in Chapter 2, IN625is a Ni-base superalloy commonly used in applications where high temperature is reached, like jet-engines and steam turbines. Cr3C2 was selected as reinforcement due to its high reactivity (compared with other ceramics) that permits to react with the metal phase during the coating process promoting secondary phases, like carbides, that modify the initial metallic matrix, improving its wear behaviour.

This research work includes several novelties respect to those presented by Betts [9] in different points. Firstly because the laser employed during theLC was a High-Power Diode Laser (HPDL) and secondly because the tribological study was performed on a micro-scale. To the author knowledge, no works about the microscopical wear performances of IN625andMMC

coatings were performed at the moment. In particular, no studies about the variation of the local wear response and the mechanical properties with the exposition at high temperature.

The objectives of these research work could be divided into the following points:

1. Select a combination of process parameters to generateIN625−Cr3C2 and IN625coatings onto a steel substrate byLC.

2. Evaluate the effects to introduce Cr3C2 ceramic particles into the IN625in terms of:

• microstructure;

• mechanical properties;

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3. Evaluate the effects of the exposition at high temperature isothermal oxidation on theIN625and the matrix of theMMCcoatings, studying each one of the items listed at point 2.

4. Study the correlation between the modifications of the microstructure and the variations of the mechanical properties and local wear beha-viour, due to the introduction of theCr3C2 ceramic particles and to the exposition at high temperature.

Bibliography

[1] D. Zhang and X. Zhang. Laser cladding of stainless steel with Ni-Cr3C2 and Ni-WC for improving erosive-corrosive wear performance. Surface

&Coatings Technology, 190:212–217, 2005.

[2] P. Hugh. Handbook of refractory carbides and nitrides. William Andrew Applied Science Publishers, New Jersey, USA, 1996.

[3] S. Söderberg, M. Sjöstraqnd, and B. Ljungberg. Advances in coating technology for metal cutting tools. Metal Powder Report, 56 (4):24–40, 2001.

[4] R. Wei, J.J. Vaja, J.N. Matossian, and M.N. Gardas. Aspects of plasma enhanced magnetron-sputtered deposition of hard coatings on cutting tools. Surface & Coatings Technology, 158-159:465–472, 2002.

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[6] Y.P. Kathuria. Nd-YAG laser cladding of Cr3C2 and TiC cermets. Surface & Coatings Technology, 140:195–199, 2001.

[7] Z. Liu, J. Cabrero, S. Niang, and Z.Y. Al-Taha. Improving corrosion and wear performance of HVOF-sprayed Inconel 625 and WC-Inconel 625 coatings by high power diode laser treatments. Surface & Coatings Technology, 201:7149–7158, 2007.

[8] L. Wang, J. Zhou, Y. Yu, C. Guo, and J. Chen. Effect of powders refinement on the tribological behavior of Ni-based composite coatings by laser cladding. Applied Surface Science, 258:6697–6704, 2012.

[9] J.C. Betts. The direct laser deposition of AISI316 stainless steel and Cr3C2 powder. Journal of Materials Processing Technology, 209:5229– 5238, 2009.

[10] G.L. Goswami, S. Kumar, R. Galun, and B.L. Mordike. Laser cladding of Nickel based carbide dispersion alloys for hardfacing applications.

Lasers Engineering, 13:35–44, 2003.

[11] J. Nurminen, J. Näkki, and P. Vuoristo. Microstructure and properties of hard and wear resistant MMC coatings deposited by laser cladding.

International Journal of Refractory Metals& Hard Materials, 27:472– 478, 2009.

[12] H. Luo, K. Gong, S. i, X. Cao, X. Zhang, D. Feng, and C. Li. Abras-ive wear comparison of Cr3C2/Ni3Al composite and Stellite 12 alloy.

Proceeding of Sino-Swedish Structural Materials Symposium, 2007.

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M

any industrial applications are developed in aggressive environments characterized by the use of high working temperatures, high gradients of temperatures, high pressures, high stresses on single components, by the presence of an oxidizing or corrosive atmosphere, and also by the presence of hard particles, generated directly into the process or becoming from the external environment, that must generate erosion or damages by impact. Some examples of industrial sectors where it is possible to find these types of processes include aircraft engines, vapour turbines, industrial gas turbines, carbon converters, oil and gas industries and nuclear plants. The concept underlying all these utilisations is the generation of a big amount of heat due to high temperature. For this reason, materials able to work at high temperatures under stress conditions are required to fulfil the required conditions of performance and durability.

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In addition, the industrial sectors try to move every day towards higher performances and longer durations, that corresponds for example to higher working temperatures and so to harder working conditions. To accomplish these goals it is necessary, in parallel with the improve of new technologies, to develop new materials that are able to work in extreme conditions. The consequence is an increase of prices of the raw materials and the employed technologies. Generally, to reduce the manufacturing costs, after the structural design of an engineering component that must work in a high temperature environment, it is possible to implement a superficial treatment to avoid these types of degradation. In most cases, the generation of a superficial coating that act as a barrier between the material and the aggressive atmosphere is the selected solution.

In the present work, two types of high performance materials are used as coatings to improve the durability at high temperature of a steel substrate. The first one is a fully metal Inconel 625 coating while the second is a Metal Matrix Composite coating obtained by the combination of a Inconel 625 and

Cr3C2 ceramic particles. Both the coatings are applied by Laser Cladding. In this chapter, a description of the superalloys, the Metal Matrix Composite, and the techniques used to generate the coatings will be provided.

2.1

Superalloys

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Superalloys are best known for their applications in components of aircraft-turbines. In the 1800s, relatively advanced steam turbines were used in industrial electric power generation in Europe while in the United States this technology became of interest only around 1905. In this period, advances in aerodynamic theory, literally changed the way to design in England, Germany and Italy, and brought to the concept of the jet-engine powered aircraft. This new technology made essential the research of new materials that could operate at higher temperature. In the 1910-1915, the austenitic stainless steel was developed and itsγ Face-Centred Cubic (FCC) structure, became the first stone that brought to the generation of the super-alloys. In 1940s, their improvements experienced a rapid progress, first in military applications and then in other industrial sectors where gas turbines were needed. New alloys compositions and new fabrication processes were elaborated [3]. Even if the development of the aircraft turbines permitted the appearance of the superalloys, this is not the only application of these alloys. They are also employed in heat exchangers, furnaces, industrial turbines, chemical- and petroleum-processing equipment, nuclear-power systems and other applications where a high temperature are reached and could deteriorate the properties of the employed materials. Superalloys constitute in fact the upper limit of properties of the metals.

The superalloys are commonly produced by cast and wrought, even if recently also powder-metallurgy techniques are used [1–3].

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other elements were added for specific mechanical and chemical effects. Although some elements are not used today in superalloys, most of the early alloys, like Nimonic 80A, A-286 and X-40 are still in use [3]. The austenitic FCCstructure of the superalloys’ matrices permits to generate alloys with different elements. In addition, this crystallography structure posses high ductility and promote the precipitation of strengthening phases. Generally, the strengthening mechanisms could be classify as solid solution, precipitation, and carbide hardening. Usually, the superalloys owns at least two of these mechanisms. At the same time, the presence of hardening phases in their composition, make some of these alloys difficult to weld, limiting in this way their applications. Controlling the composition and by the use of heat treatments, it is possible to change their properties [2, 3].

The microstructure of the superalloys consist essentially in a γ matrix and secondary phases likeM C, M23C6,M6C andM7C3 carbides, in all the superalloys types,γ0 ordered L12 N i3Al and N i3(Al, T i), γ00 ordered D022

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2.1.1

Inconel 625

Inconel 625 (IN625) is a Ni-base superalloy used in industrial applications from cryogenic to about 950oC for its high strength, excellent machinability and corrosion resistance.

Ni-base superalloys are solid-solution, precipitation, or particle/oxide-dispersion strengthened. Nickel is an excellent base for alloys because it is possible to combine it with different elements maintaining its ductileFCC

structure. The cost of these alloys is higher than that of stainless steels. For this reason, their use is limited when stainless steels are not suitable or when product purity and safety are critically important [3–5]. Nickel is resistant to alkalis (hot or cold) and diluted non-oxidising inorganic and organic acids, to oxidation in air up to 800-875oC, it does not suffer stress-corrosion cracking except at high concentrations of alkali or in fused alkali and it is passive to many aerated aqueous solutions, but the passive film that grown is not very stable. At the same time, nickel is not resistant to oxidizing acids like

HN O3, oxidizing salts like F eCl3, aerated ammonium hydroxide, alkaline hypochlorites, seawater and sulphur-containing reducing environments (T

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Table 2.1: Common ranges of principal alloying elements in Ni-base superalloys [2].

Element Range, wt.%

Chromium 5-25

Molybdenum, tungsten 0-12

Aluminium 0-6

Titanium 0-6

Cobalt 0-20

Niobium 0-5

Tantalum 0-12

Rhenium 0-6

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Table 2.2: Chemical composition limits for IN625alloy [6].

Element Limit, wt.%

Nickel 58.0 min.

Chromium 20.0-23.0

Iron 5.0 max.

Molybdenum 8.0-10.0

Niobium (plus Tantalum) 3.15-4.15

Carbon 0.10 max.

Manganese 0.50 max.

Silicon 0.50 max.

Phosphorus 0.015 max.

Sulfur 0.015 max.

Aluminium 0.40 max.

Titanium 0.40 max.

Cobalt 1.0 max.

With the name INCONEL, trademark of Special Metals Corporation [6], it is possible to round up a series of austenitic Ni-Cr alloys that were developed in 1940s. Among these, there is the Inconel 625, also known as Alloy 625. Its chemical composition’s limits are detailed in Table 2.2. The strength of the IN625 come from the presence of molybdenum and niobium in a nickel-chromium matrix. This combination of elements also generates a superior resistance to corrosion environments, oxidation and carburization [6–8]. Due to its characteristics that combine yield strength, creep strength, fatigue strength, oxidation and corrosion resistance, the

IN625alloy finds numerous application in different industrial sectors like aerospace, chemical, petrochemical, marine, nuclear, power generation [8–13]. In the aerospace industry, it is possible to find IN625in turbine shrouds, spray bars, hydraulic tubing, armouring and thrust reversers systems [6,13].

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phases were found studying its behaviour during service at high temperature. It was studied that an exposure at temperatures below 600 oC for times larger than 50,000 hours generates a considerable deterioration of the alloy. On the other hand, shorter ageing treatment above service temperature considerably increase its toughness [12]. In addition, as described in Section 2.1, various forms of carbides can also form varying time and temperature of ageing [9, 10, 13].

As mentioned before, the price of Ni-base superalloys, includes theIN625, is higher than the stainless steel one. To feature with the high properties of this alloys and at the same time limit the costs, a possible solution is to use them as coating onto a cheaper structural material. In Section 2.3 the different techniques commonly used to generate a coating were described.

2.2

Metal Matrix Composite

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In this document, the attention was focused oncarbides-reinforced MMC, where the ceramic phase was directly introduced in the matrix during a coating process. To be of interest in engineering application, a carbide must be hard, refractory and chemical inert [1]. For these reasons, just two family of carbides actually found engineering application: covalent and transition-metal carbides. The first group include the covalent carbides of silicon and boron. The silicon carbide (SiC), thanks to its properties is extensively used to produce abrasive and cutting tools, component of high-temperature gas turbines, bullet-proof armours, brake discs of vehicles, lighting arresters in electric power system, LEDs, etc. The boron carbide (B4C) is a very hard ceramic that is commonly used as abrasive material for fine polishing and ultrasonic grinding and drilling, in the refractory industry, to manufacture heat-resistant parts like spray and blasting nozzles, as tank armour and anti-ballistic plating, as well as other numerous indus-trial applications. Remarkable is its use in nuclear applications as radiation shields and moderators [1, 22]. The second one is the class of the carbides derived from transition metals of Groups IV, V, and V I like titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, and tungsten [1, 22, 23]. All the carbides of this group could be employed to manufacture cutting tools, wear-resistant surfaces, and as ceramic phase for cermets and MMC components. In addition, thanks to their specific properties, each one of these carbides found particular uses in different industrial sectors. I.e., the titanium carbide (TiC) is used as heat shield for spacecrafts while the zirconium and the niobium carbides (ZrC and NbC) are used as refractory coatings in nuclear applications.

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as coating could contain the costs reducing the amount of material used. The carbide coatings are characterized by a high level of porosity if they are sprayed to obtain a coating. This factor reduce their properties like the service temperature and the erosion resistance. To optimize the benefits derived by the use of carbides, it is possible to spray it with a metal matrix that act like a binder, improves the interparticles cohesion and the bonding with the substrate, and generate in this way a Cermet or a MMCcoating. The carbide reinforcement used in this research work is a chromium carbide which characteristics will be described in the following Chapter and belonging to the transition-metal carbides group. Chromium carbide coat-ings are of great interest due to their high-temperature resistance, thermal stability, wear and oxidation behaviour. In addition, the chromium carbide has a thermal coefficient of expansion similar to steel. As it is commonly used to coat steel parts, this property permits to reduce the residual stresses at the coating-substrate interface. Examples of applications of chromium carbide are piston rings, high friction coatings, rod mandrels, hot forming dies, machine parts, hydraulic valves, wear protection of aluminium parts and grain refinement in carbide cutting tools.

2.3

Surface engineering

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Different way to modify the superficial properties were developed and could be classified as follow [24]:

• metallurgically;

• mechanically;

• chemically;

• adding a surface layer or coating.

Furthermore, in addition to structural and functional aspects, also economic properties as price and availability, play very important roles during the selection of the materials for a particular application. To limit the prices of the manufacturing processes and also the costs of the final products, it is important to design a component trying to avoid wastes [25]. The use of superficial treatments permits to reduce the amount of high quality (and so expensive) materials limiting their use just for a superficial layer and using a cheaper bulk material, diminish in this way the manufacturing prices [24,26].

In this research, we focus our attention on the last of the solutions listed before: thesuperficial coatings. The coating process of a substrate consists in the intentional creation of a new layer with different composition and improved properties with respect to the underlying metal [1].

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In addition, the selection of one coating technique instead of another could benefit the service life of the component and permits, for instance, its reparation in case of damage, and so reduce its life costs [27].

Metals and ceramics could be employed as coating materials, individually or as mixtures, but their properties could limit the range of available techniques of application.

It is possible to find in literature different ways to classify the disparate coating processes, as a function of the nature of the filler material, of the properties of the final surface, of the energy source, etc. Considering just the metal and the ceramic coatings, an example of grouping is the one shown by Grainger and Blunt [27] that divided the different methods inwelding,

thermal spraying,electrodeposition, and vapour deposition.

Welding. These techniques permit to obtain coatings with thicknesses of

the order of the millimetres, greater than the obtained with other methods. The adhesion provided is very high due to a metallurgical bond generated between the coating and the substrate during the process. At the same time, a dilution1 with the substrate material is inevitable, but the extension of the intermixed zone could be controlled varying the welding parameters. Due to the high thickness and the irregular superficial morphology, it is possible machining the surface of the coating after its deposition. The common defects that could be found in a welded coating are contraction cracks, that usually do not affect the separation of the coating from the substrate, grinding cracks and subsurface cracks, that could generate spalling under mechanical or thermal stress. The levels of porosity and inclusions are usually very low compared with those obtained with other techniques.

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This family of coating methods groups the following processes:

• arc welding with filler material (Gas Metal Arc Welding,GMAW; Gas Tungsten Arc Welding,GTAW;. . .);

• oxyacetylene welding (OA);

• powder welding (PW);

• electroslag welding (ESW);

• resistance welding;

• friction surfacing;

• explosive cladding;

• laser cladding (LC).

Thermal spraying. The big amount of spray techniques use filler material in form of powders with morphologies, sizes and size distributions of the particles that depend on the nature of the material and the employed method. An alternative way is to use filler material in form of wire [28].

The spray process are made up by three principal steps:

1. create the filler material with the correct composition in form of powder or wire;

2. supply enough kinetic and thermal energy to generate a confined flux of high energy particles;

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The particles of the filler material, molten or not, deform plastically once they impact against the substrate or other particles previously deposited. In part due to the impact energy, the sprayed particles form a cohesive bond between themselves and the substrate. If all the process energy is kinetic and there is no thermal contribution, it is possible to talk about Cold Spray (CS) technique (also known as Cold Gas-Dynamic Spray method,

CGDS) while, if there are both the contributions of energy, is common way talk about Thermal Spray (TS) techniques.

The different spraying techniques can be also classified in various forms, for example as a function of the type of carrier gas, as a function of the price, etc. Below, the mainTStechniques are classified as a function of the type of heat source employed to obtain the thermal energy [28]:

• flame generated by flammable gas (Flame Spraying,FS; High-Velocity Oxy-Fuel Spraying, HVOF);

• detonation of combustible gas (Detonation-Gun Spraying,D-GUN);

• plasma produced thanks to an electric discharge (Atmospheric Plasma Spraying,APS; Controlled-Atmosphere Plasma Spraying,CAPS; Va-cuum Plasma Spraying, VPS);

• electric arc (Arc Spraying,AS).

A graph showing the ranges of temperatures and particles velocity for the most commonTS techniques is reported in Figure 2.1 [29].

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Figure 2.1: Temperature vs. particle velocity ranges for Thermal Spray techniques [29].

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alloys can be electrodeposited. Electrodeposition is also used to repair damaged parts and to built directly a component (electroforming).

For engineering purposes, two methods of electrodeposition are available:

• vat plating;

• selective plating.

Vapour deposition. In this category are grouped these methods that permit to obtain a coating by condensation or reaction from the vapour phase. It is possible to define two main techniques:

• physical vapour deposition (PVD), including the sputtering;

• chemical vapour deposition (CVD).

The first one is used to obtain relatively thin films, with thickness between 10−7 and 10−4 m. The latter is used for both thin films and coatings of over 1 mm. All vapour deposition process involve a treatment in a special dedicated chamber. This fact could limit the size of the objects to be coated. In addition, high temperature treatments are needed to obtain the reactions that permit the creation of the superficial layer. For this reason, the substrate must be compatible with the process.

Thermal spraying are the most common techniques used to process

MMCcoatings, in particular High-Velocity Oxy-Fuel Spraying (HVOF) and Plasma Spray (PS) [18, 19, 30].

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chamber with oxygen. An ignition is produced and the exhausted gases are set free in the atmosphere. The filler material in form of powder is axially introduced in the gas flux and the entire system is refrigerated with water. The main parameters that control the jet emerging from the torch nozzle are the pressure in the combustion chamber (0.3-1MPa up to 4 MPa), the profile of the nozzle-barrel (that influence the pattern of the flame flow and its velocity), and the stoichiometry fuel/oxygen. At the exit of the torch, the sound speed is overreached and the gas flux has the characteristic of a supersonic flow. The porosity of the coatings obtained in this way is close to the 1% and thickness around 100 to 300 µm with a coating/substrate bonding around 40 to 80MPa are commonly obtained [28].

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Recently, the use of welding methods is increasing. In particular, lasers methods are every day more employed due to the benefits obtained with these techniques, in terms of higher adhesion coating/substrate, less porosity, and cost reduction [21, 31–34]. In the following Section, a description of the superficial modification by laser will be done, focusing the attention on the Laser Cladding (LC) technique.

2.4

Laser surface modification

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Figure 2.2: Scheme of the mechanism of population inversion of species.

industrial sectors and the laser processing became another means of manu-facturing. Some examples of well known manufacturers that began to use the laser technology in 1990s are General Electric, Pratt & Whitney, Allied Signal, Rolls-Royce, Allison, Solar, MTU, Fiat, Toyota and Mercedes-Benz. At the present days, the research works on this field are focused on the way to create smaller, cheaper, more efficient, and flexible laser systems [39, 40].

The word LASER is the acronym ofLight Amplification by Stimulated Emission of Radiation. As concluded by A. Einstein, the photons (group of wave energy that form the light) interact with species (atoms, ions, and molecules) by absorption, and spontaneous or stimulated emission. In stimulated emission of light, a photon collides with another excited species generating the premature release of another photon. It brings to a phenomena known aspopulation inversion of species between two energy levels (Figure 2.2).

As the photons have the same phase, frequency, and polarization, the light emitted by lasers is:

• high monochromatic: the laser beam consists of a narrow range (almost

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relation): Ex - E0 = h · ν = h · c · λ−1, where h is the Planck constant,ν the frequency,c the speed of light, andλthe wavelength;

• high coherent: the laser light has a constant phase difference in two

or more waves over time;

• high directional: this characteristic permits to the laser light to travel

in a single direction within a narrow cone of divergence;

• sharply focused: the focus spot of laser light is such small that brings

to a very high intensity.

The laser light could be formed by a continuous wave, a pulse, or a train of pulses. The power generated could vary between some milliwatts to several kilowatts with peak of the order of the gigawatts [39]. The simplest way to generate laser emission is between two energy levels E2and E1 that represent respectively the ground and the excited states. By the way, obtaining useful laser light in this way is not easy and for this reason industrial laser systems are based on three and four energy levels. In a three-level system, laser emission occurs between levels E3 and E1, where E3 represents the upper laser level (Figure 2.3). If the lower laser level corresponds to the ground level, the output is limited to pulsed operation.

A more easily laser transition could be achieved if the lower laser level does not correspond with the ground level (Figure 2.4). The laser transition in this case is to an intermediate level, usually unpopulated, and for this reason a four-level laser system could operate in continuous wave mode.

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Figure 2.3: Three-level laser system.

the circulating one, and so the output power.

From a practical point of view, a laser system requires at least four components to work:

1. active medium (gas, liquid, or solid);

2. excitation source;

3. optical cavity;

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Figure 2.4: Four-level laser system.

In addition, a laser requires a power and control systems, a cooling system, and an interface for operation.

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Gas lasers. The lasers that belong to this group utilize a gas or mixture of gases as active medium. The excitation is obtained applying a current through the gas and they could operated in continuous and also in pulsed waves mode.

It is possible to classify the lasers of this family as a function of the nature of the employed gas: neutral-atom gas (i.e., HeNe), ionized gas (i.e., HeCd), and molecular gas (i.e., CO2). Among them, the CO2 one is widely adopted in industrial applications. It has a wavelength of 10.24 µm and an output power that can exceed the 45 kW. The problems connected to this type of laser system are the high maintenance costs and its low efficiency (function of the temperature). By the way, its beam quality and focusability is better than the other lasers with the same power. In addition, it is well absorbed by organics, glass and ceramic materials, and it is colour independent.

Excimer lasers. The term excimer is used to refer to an “excited dimer”.

The active medium of these lasers is composed by molecules formed by an inert gas atom and a halogen gas atom (i.e., KrF, XeF, ArCl). The excimer laser is generated by the combination of two identical atoms or molecules, one in the ground state and the other excited. The excitation could be reached with an electric discharge or an electron beam (E-beam).

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Liquid dye lasers. Dye lasers are unique due to the use of a liquid as active medium. The wavelength of the output laser light depends on the particular dye used. They are commonly pumped by other lasers and their most useful feature is their tuneability. Dye lasers are relatively hazardous because of the toxicity of some employed liquids and the high voltage power supply required to excite the active medium.

Solid-state lasers. The solid-state lasers use a solid crystalline material as active medium and are optically pumped with flash lamps or diode lasers. The first laser invented was a solid-state one [35]. In this first version, a ruby rod (chromium-doped aluminium oxide) with mirrors on both ends, was pumped with a helical xenon flash lamp. Currently, solid-state lasers are quite similar to the first one and usually use neodymium doped materials. Some examples are the Nd:YAG (Y3Al15O12), the Nd:YVO4, and the Nd:Glass. In these lasers the excitation of the active medium was obtained introducing a positive ion in the structure of the ceramic active medium. The result is a population inversion and the emission of laser light. The strongest output wavelength is close to the infrared (1064

nm) and so invisible. These lasers can operate in pulsed, continuous and quasi-continuous wave modes. The Nd:YAG and the Nd:YVO4 occupy an important role among the high-power laser systems (i.e., 4kW with 1064

nm) currently employed.

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The first high-power fibre laser (2 kW) was developed in 2003 and currently they are progressively replacing other high-power lasers like Nd:YAG and CO2 ones, principally due to their higher beam quality. Fibre lasers are an efficient and compact solution for micromachining and are easily integrated into industrial solution for different reasons like their high electrical efficiency, no water cooling required, high quality beam, optimized pulse duration, and maintenance-free operation.

Semiconductor lasers. They are also known as “diode lasers”. In these

lasers, the active medium is a nearly microscopic solid-state device, similar to a Light Emitting Diode (LED), that may built into arrays. These elec-tronic components are made by semiconductors of group III-V compounds2 (i.e., GaAs, AlGaAs, InGaAs, InGaAsP). The electron transition from the valency to the conduction band in these doped semiconductors, has as result a population inversion. The two bands stay at different energy levels and the wavelength of the emitted laser light depends on their energy gap (Planck-Einstein relation). Spontaneous and stimulated emission occurs in a narrow region of thep-n junction3 when the electrons of the conduction band occupy the holes in the valency band. To obtain a stimulated emission, an electric current must be supplied. The optical cavity in these lasers is formed by separate two opposite sides of a semiconductor wafer to form a Fabry-Pérot lasing cavity4 [41].

The first diode laser was develop in 1962 by General Electric and IBM,

2

Combination of elements of the third group of the periodic table with elements of the fifth group.

3 In a doped semiconductor, it is the interface that divide the two materials, one with an

excess of electrons (nstate), and one with a loss of electrons (pstate).

4 Optical cavity delimited between two flat surfaces that partially reflect with a high

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using ap-njunction GaAs diode. In 1970s, in USA and in the Soviet Union, the operation of a diode laser in continuous wave mode was demonstrated. Only in 1980s the semiconductor lasers became more economical and so more available. The principal properties that characterize the diode lasers include a wide spectrum band (2-20nm), a large beam divergence (up to 40 half-angle), non-symmetrical beam distribution (2.5-6 times difference between two orthogonal axes), and lower energy intensity per area. Due to these properties and also for their compactness, low power consumption, and low cost, the diode lasers found in the recent years new and different applications in different industrial sectors like chemistry, bioanalysis, de-fence, medicine, telecommunication, agriculture and environment, space, and material processing, where they are commonly used for soldering, weld-ing, brazweld-ing, hardenweld-ing, and cladding. In addition, semiconductor lasers are also employed as pumps for solid-state lasers [41, 42].

The new generation High-Power Diode Laser (HPDL) have power out-puts up to 6kW with rectangular beam profiles that are larger than those of Nd:YAG and CO2 lasers. For these reasons, in addition to the other common properties for the semiconductor lasers, it is possible to consider theHPDLas a competitive alternative choice [42, 43].

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is modify just some microns of depth. Other parameters that could play a role in the laser system selection process are the sample dimensions, the characteristics of the processed materials, the availability, and the costs.

During the last decades, different engineering applications for laser sys-tems were developed. It is possible to divide the different laser techniques in

thermal andathermal processes. In thermal processes, three principal mech-anisms could be generated due to the interaction between the laser beam and the treated material: heating, melting and vaporization. Examples of these methods are surface melting-hardening, cladding, conduction joining, cutting, keyhole welding, marking, and thermal machining. Athermal pro-cesses are those where the interaction between beam and material generate a resonant transfer of energy with no change in temperature. Beams with a pulse duration of the order of 10−15 s are able to generate mechanisms of interaction that do not follow the laws of the classical thermal conduction. Examples of laser athermal techniques are the laser printing, the haemato-porphyrin derivative, the photochemical annealing, the stereolithography, the optical lithography, the photorefractive keratectomy, and the shock processing [39].

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materials, etc. [45]. Another advantage to use laser systems is that the laser light could be transported through fibre optics which permits to reach also the most remote corner of the treated component.

2.4.1

Laser cladding

In the last two decades, laser systems were found to be useful in the deposition of metallic, ceramic, and mixture of both materials onto metal-lic substrates obtaining coatings with excellent wear and erosion proper-ties [15, 16, 21, 26].

Laser Cladding (LC) is one of the most important laser surface modi-fication techniques. The most important limitation to its use in industrial applications was connected to the big variety of conventional and more fa-miliar thermal spray techniques (see Section 2.3). Despite it, LCfound and currently continue to find more space in industrial sectors like automotive, aerospace, petrochemical, oil and gas, etc., to generate coatings able to resist to oxidation and corrosion environments in addition to wear, friction, and erosion phenomena [8, 15, 16, 21, 26, 45–47]. This increase success is due to the characteristics of the obtained coatings, like low porosity, sporadic imperfections, and strong bond with the substrate. This last property is favoured by the energy transfer toward the substrate, that work as heat sink. In this first layer of the substrate material a Heat Affected Zone (HAZ)5 with a finer microstructure was therefore generated [39, 40]. In addition, the use ofLCpermits to reduce the post-coating procedures time and cost, and improving the controllability of the process. Table 2.3 present a comparison between some characteristics of the coatings obtained by LCand the most

5Zone of the substrate material where the peak temperatures generated during a welding

Figure

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Referencias

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