EXTRACCIONES Y DEMOLICIONES

Boron nitride (BN) is a synthetic polymorph material. According to Heath (1986), boron and nitrogen can be formed into a compound by the chemical reaction shown in Equation 2–1.

BCl3+NH3 = BN+3HCl ... (2-1)

Boron nitride can be typically found in four crystalline forms, according to different pressure and temperature conditions. The forms are: cubic (cBN), hexagonal (hBN), rhombohedral (rBN), and wurtzitic (wBN). The denser ultra-hard forms of boron nitride are the cBN and wBN (Liu et al., 1995).

Boron nitride (BN) is a soft, slippery, friable substance and exhibits a hexagonal structure. Under high temperature and pressures (temperatures in the range 1400–1750°C and pressure of the order of 5-8 GPa), hBN can be transformed into cBN as shown in Figure 2–8(Heath, 1989).

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Figure 2-8 Transformation of hexagonal boron nitride to cubic boron nitride (Adapted from Heath, 1989)

Solvents or catalysts, mostly metals, are generally added to hBN in order to increase the transformation rate and also effectively reduce the required pressure and temperature to a more easily attainable level of about 6 GPa and 1,500°C, respectively. The cBN can be liberated and recovered for subsequent processing after the grains grow, by dissolving unwanted matrices (Heath, 1989; Lopez et al., 2011).

cBN tool is a superabrasive material which is second in abrasive resistance and hardness only to diamond. cBN is produced under high temperatures and pressures. cBN tools also have an important advantage over diamond tools, namely chemical inertness with steel (Huang et al., 2007).

cBN has very good mechanical properties which are attributed to its crystalline structure and its covalent link (Lopez et al., 2011). The physical properties of cBN are as follows: density, 3.48 g/cm3, thermal conductivity 13 W/cm°C at room temperature, hardness, 4,500 HV, Young‘s modulus, 71×103

N/mm2, and thermal expansion 4.7×10-6/°C from room temperature to 800°C.

PcBN is a composite material consisting of cubic boron nitride (cBN) grains in a binder matrix. Commercially manufactured PcBN tool products are generally called CBN tools; they are available at variable cBN contents with some additives. PcBN materials are categorized into two, namely, high cBN content material or low cBN content. High cBNcontent grades contain the cBN grains with a metallic or ceramic binder(s) (such as cobalt, W-Co-Al, Ti-Al and Al ceramic) and have approximately 0.8–0.95 volume fraction of cBN grains. Low cBN content grades can contain from 0.4–0.7 volume fraction of cBN and have a ceramic based binder such as Titanium carbide (TiC), Titanium nitride (TiN) or AlN (Harris et al., 2004,

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Lahiffet al., 2007, Huang et al., 2007, De GodoyandDiniz, 2011). High cBN content and Co matrix grade is recommended for interrupted cutting of iron castings, while for finishing operations, low cBN content and ceramic matrix are recommended.

Research to date has found that low cBN content materials provide the best performance in hard turning in terms of tool life and surface finish (Lahiff et al., 2007)

During the sintering of PcBN cutting tool materials, with more than 0.8 volume fraction cBN grain, cBN grains form a skeletal structure with considerable cBN phase contiguity. But, the contiguity of cBN is limited when the volume fraction of cBN grains is lower (0.4–0.6) (Can and Andersin, 2006).Ultrafine powders of a secondary hard phase are purposely incorporated in the binder phase as grain growth inhibitors, to prevent or reduce grain growth of the binder phase during the high temperature–ultra high pressure process when the volume fraction of the cBN grains in the PcBN cutting tool material is less than 0.7.

For applications such as turning, milling and drilling of pearlitic iron castings, both grey and ductile, PcBN is normally recommended. PcBN is not suitable for machining ferritic iron castings, because of its high reactivity of ferrite with cBN, which degrades the cBN owing to diffusion of boron within the ferritic matrix (Lopez et al., 2011).

PcBN cutting tools are fabricated in the form of substrate backed or master blanks compact structures. The making of PcBN cutting tool material consists of sintering randomly orientated cBN grains, typically mixed with various binder phase precursors, as mentioned above. The latter is necessary because sintering of highly pure cBN compacts is generally difficult because of the predominant covalent atomic bonding of cBN, thus requiring high temperature and ultra-high pressure conditions for full densification (Can and Andersin, 2006). To overcome this problem, binders are used as sintering aids for obtaining fully dense PcBN cutting tool materials (Heath, 1989).

In the making of PcBN cutting tool materials, composites or compounds of Group 4, 5 or 6 transition metals are most frequently used as binder phases. Among these particular compounds, TiC and TiN exhibit the highest chemical activity towards cBN material (Benkoet al., 1999; Benkoet al., 2001). Among binder phases that are often used in the

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synthesis of PcBN cutting tool materials TiC with Al compounds, TiN with Al compounds, Al (AlB2, AlN, α AlB12), W–Co and Al compounds are commonly used (Harris et al., 2004).

During this high temperature–ultra high pressure sintering, new phases are formed as a result of chemical reactions between cBN grains and the sintering binder. In selection of the appropriate binder phase, thefundamental understanding of their mechanisms of formation and prediction of these new phases is of critical importance (Benko et al., 1999). The performance of PcBN cutting tool materials is dependent upon the bulk mechanical properties and the interactions of microstructural constituents of the tool and the work material can be vital to cutting tool performance (Chou et al., 2003).

Once PcBN is produced, the master blanks or substrate backed structures are cut into smaller blanks before being ground into shapes and sizes as tips for cutting tool inserts. PcBN cutting tools are available in either solid form or tips brazed to the solid blank (usually carbides) (Liu et al., 1995). The brazed tips result in a stronger cBN blank and allow more of the generated heat to be absorbed (Grzesik, 2009).

PcBN is an ideal cutting tool material for machining iron-based workpiece materials, most especially hardened steels, where excellent surface finish is required, but the cost of the cutting tool is an important consideration in a production environment.

In the literature, parameters such as the cutting edge geometry, cBN content, coating type, grain size of cBN, type of binder, use of coolingmethods and variation in cutting parameters have significant influence on cBN tool performance (Konig et al., 1990; Lin and Chen, 1995; Chou et al., 2003) as indicated in Figure 2–9.

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Generally, PcBN is normally recommended for hardened materials with hardness up to 70 HRC to generate surface finish down to Ra of 0.3 μm. Low content cBN is better suited for machining owing to better shock resistance, wear resistance and chemical stability, while high content cBN is more suitable for hard cast-iron and high-temperature alloys owing to its toughness.