Diseño del experimento

A new instrument for nano-scale to micro-scale friction and wear measurements was developed by Dvorak in 1998 (Dvorak, 1998). The nano-tribometer is based on a scanning force microscope design developed at NIST (National Institute of Standards and Technology, USA). Interferometers and capacitance gages are used in a closed-loop digital control system to accurately determine and control the position

and motion of the sample. The instrument incorporates several design features, including three-axis feedback of the sample position, a position-sensitive PIN photodiode for large dynamic range force measurements, and available various cantilever probes. The force head can measure continuous changes in both normal and lateral forces over three orders of magnitude within the range of nN to mN. The nano-tribometer can be used at a constant normal load of 40 nN and a constant speed of 14.4 µm/s for each test, but little experimental data is currently available.

A ball-on-flat reciprocating micro-tribometer was designed by Le in 2005 at Cambridge to measure the friction coefficient for an aluminium alloy strip sliding against a Ø 3 mm steel ball (AISI 52100) as shown in Figure 2.20. The normal load is applied in the range of 0.1-10 N by a flexible wall soft spring made from a single block of aluminium alloy and the spring only allows vertical displacement with a stiffness of 7 N/mm. The sample holder is fixed to a thin-walled octagonal ring aluminium alloy load cell fitted with strain gauges in full-bridge configurations. The sample stage can be driven by a D.C. mini-motor at reciprocating speeds of 0.02 mm/s, 0.1 mm/s and 0.5 mm/s, with a sliding distance of up to 10 mm. This micro-tribometer was used to assess the effectiveness of various boundary additives and the development of a transfer film in engineering applications of aluminium alloys and their metal forming processes.

Figure 2.20 Schematic of the micro-tribometer (Le, 2005)

Recent research efforts by O. Mollenhauer in 2006 at the Ilmenau University of

technology (Germany) had focused on using a new generation of micro-tribometer, which consists of three main components including precision motion mechanisms, force transducers and fiber-optic length detection system (Mollenhauer, 2006). Figure 2.21 shows the main components marked: (1) x-y micro-positioning stage powered by stepper motors (2) reciprocating unit, (3) sample, (4) counter body, (5) force transducer, (6) fiber-optic sensors to detect normal and lateral deflections of the force transducer. The inset shows a glass spring force transducer mounted on a steel support.

The precision motion mechanisms are used to drive sample position, reciprocating motion and to adjust normal force. A double leaf spring made from photo-structured glass is applied for the force transducer to measure lateral and normal forces. In this work, friction force was studied as a function of the relative

humidity with the test conditions including 0.2 mN normal load, 50 μm/s sliding speed and 100 μm stroke. The counterbodies were 2 mm diameter steel AISI 440C balls coated with titanium carbide (TiC) with an 𝑆_𝑞 roughness of 2.8 ± 1 nm and were polished after deposition.

Figure 2.21 A picture of Mollenhauer’s micro-tribometer (Mollenhauer, 2006)

A micro-friction test apparatus was specifically developed by Qing Tao in 2007 at Tsinghua University to measure the maximum static friction, the sliding friction and the adhesion. Figure 2.22 shows the schematic diagram of the micro-friction test apparatus. The apparatus includes two cantilevers with mounted strain gauges for

the force measurement in the lateral and vertical directions, a precise vertical movement for load generation, a horizontally moving table and the data acquisition system. The force measurement range was 10 µN to 2000 µN with a force resolution of 10 µN. The movement of the vertical table created a controllable deformation of the vertical cantilever to generate the required normal loads. The horizontal table was driven by a stepped motor with the horizontal speeds of 10 μm/s - 400 μm/s. The data acquisition system uses a voltage amplifier, a wave filter and a computer with an analog-digital converter (ADC) for the measurement of the normal load and the friction force. The measurement experiments were carried out using a 0.7-mm diameter steel ball as the counter-body to study the micro-friction behavior of a single crystal Si (100) wafer and a TiB2 thin solid film as a function of the load, sliding speed, and rest time in terms of the micro-friction and adhesion.

A reciprocating ball-on-flat tribometer (Kosinskiy, 2012) was developed by Mikhail Kosinskiy in 2012 at Technische Universität Ilmenau to evaluate the tribological behavior such as adhesion, friction and wear of materials and coatings in vacuum and controllable atmospheres on a microscale with applied normal loads of 0-50 mN and sliding speeds of 10-180 µm/s. For this tribometer, a flat sample with 5-15 mm in linear dimensions and heights ranging from to 0.5-3 mm, is mounted on the 2D actuator that provides the horizontal and vertical motion. A ball with a diameter in the range of 1-3 mm is attached to the force sensor. The sensor itself is fixed onto the base unit of the tribometer. During the measurement, a cantilever with an attached tip is fixed and a sample performing relative motion by a piezo-actuator. The force sensor consists of a couple of springs, which deforms in the horizontal and the vertical direction. These deflections are monitored by an optical detection system consisting of small square aluminum coated silicon mirrors, which are diced from the Si (100) wafers, along with a couple of commercial miniature laser interferometers (SIOS GmbH, Germany) mounted outside the vacuum chamber.

Friction experiments involving a Ø 2 mm Si ball sliding against a flat Si (100) wafer (thickness: 300–350 µm) and SiO2 layer with a thickness of 2 nm, at a sliding speed of 180 mm/s and a normal load of 50 mN, demonstrated the validity of the

new vacuum micro-tribometer compared with a commercially available instruments in the literature and other types of tribometers under similar conditions. Also, the CoF as a function of sliding cycles showed a stable behaviour for vacuum measurements, while there is a higher value but similar variation in CoF between the

microtribometer and the commerical avaliable instrument under ambient conditions.