K.L. Rutherford

A micro-abrasive wear test, with particular application to coated systems

A test methodology is described which permits a rotating sphere test, also used to measure coating thickness, to be used as a small-scale abrasive wear test. The ability of the test to measure intrinsic wear resistance for thin coatings is demonstrated together with a new method of analysis which allows simultaneous evaluation of the wear resistance of both substrate and coating from their combined wear behaviour in a single test. The method has been applied to physically vapour deposited TiN, TiCN and TiAlN coatings on tool steel substrates, as well as to bulk samples of metals, ceramics and soda-lime glass.

Theory and application of a micro-scale abrasive wear test

A micro-scale abrasion test has recently been developed that allows measurement of the wear resistance of the surface regions of a material. The typical penetration depth is less than 30 μm. The test uses a simple mechanical and optical system and involves rotating a hard steel sphere against a specimen in the presence of small abrasive particles. The method has been used to investigate the wear resistance of thin PVD coatings (1 to 5 µm), metallic glass ribbons, and paint films in addition to bulk samples of metals, ceramics, and glasses. The associated theory has been extended so that results may be obtained from any curved surface. This furthers its applicability to practical surface-engineered components such as twist drills, bearings, turbine blades, and biomedical prostheses. A detailed characterization of the experimental procedure has been undertaken to provide an understanding of the repeatability and sensitivity of the test. In a study of cutting-tool coatings, the wear resistance measured by this method has been shown to correlate with scratch test response and with the performance of coated end-mills in cutting tests.

Wear resistance of amorphous alloys and related materials

Abstract Amorphous alloys, and the partially or fully crystallised materials derived from them, can have properties attractive for a diverse range of applications. In some cases, their wear resistance can be of primary concern, in others, it is an important secondary property. The distinctive mechanical properties of amorphous alloys make their wear resistance of fundamental interest also. This review focuses on the influence of a variety of factors in wear testing, on the mechanisms of wear, on the characteristics of different categories of amorphous alloy, and on the effects of partial or complete crystallisation. It is shown that amorphous alloys can have very good resistance to sliding and abrasive wear. The wear resistance of related quasicrystalline phases is also considered.

The abrasive and erosive wear of polymeric paint films

Abstract Two novel methods have been used to assess the wear resistance of six different 20–70 μm thick polymer coatings on both galvanized steel and polymeric substrates. The coatings were commercially available ‘clear-coats’ which are used as the uppermost layer in automobile paint systems and are intended to provide resistance to both mechanical and corrosive damage. The first wear test involves solid particle erosion and has been developed to assess the durability of monolayer thin coatings and, in this work, has been extended for the first time to multilayered systems. The second technique uses a ball rotating in a slurry of small abrasive particles to measure the abrasive wear resistance of a material. The technique allows abrasive wear tests to be performed on a sample area smaller than 4 mm2 and examines the uppermost 30 μm of material. These techniques circumvent the most common problem experienced in performing wear tests on thin coatings, namely the measurement of very small mass or volume changes. The thickness and glass transition temperature of each coating material were measured and the mechanical behaviour was characterized by tensile tests on the coating material alone. The erosion durability of the coatings was found to be sensitive to the nature of the substrate. A correlation similar to the Ratner-Lancaster correlation for bulk polymers was identified between the wear resistance of the coatings in the abrasion and erosion tests and the energy required to break the coatings in the tensile tests. The polymer films which had been formulated for use on flexible substrates were consistently more wear resistant than those designed for steel.

Laboratory characterisation of the wear behaviour of PVD-coated tool steels and correlation with cutting tool performance

Abstract Four different characterisation tests have been used to study the wear behaviour of a series of thin (1–4 μm) PVD coatings of TiN, TiCN, TiAlN and CrN on identical M2 tool steel substrates. These include two novel tests involving micro-abrasion and erosion durability. The abrasion method provides values for the intrinsic wear behaviour of both the coating and the substrate from a single test. The erosion test establishes the critical mass of erodent particles required to impact unit area of the coating to cause removal. Standard scratch tests were also performed, and coating failure characterised by both acoustic emission signals and post-test optical examination of the scratches. The performances of the coatings in these tests were compared with those of coated end mills in accelerated cutting tests. The micro-abrasion and scratch tests both showed good correlation with the results of the milling tests; although the erosion test generally discriminated between the coatings, correlation with the other test results was poor.

Abrasive wear resistance of TiN/NbN multi-layers: measurement and neural network modelling

Abstract A recently developed micro-scale abrasion test which measures the wear resistance of surface regions of a material involves rotating a hard sphere against the specimen in a slurry of abrasive particles. The method can be applied to curved as well as plane surfaces of practical components. The wear resistance has been determined of a series of mono- and multi-layer PVD coatings (2–8 μm thick) of TiN and NbN, which have also been characterised by micro-hardness tests and X-ray diffraction. The influence on wear resistance and hardness of interlayer thickness, interlayer mixing, deposition pressure, bias and the relative proportion of the two phases has been analysed by neural network modelling.

Micro-scale abrasive wear testing of PVD coatings on curved substrates

A method is presented which enables a micro-scale abrasion test to be used to measure the wear performance of a coating over a small region, typically of millimetre dimensions, on a curved surface. The method is also applicable to studies of the wear resistance of any bulk material with a surface having complex curvature. The technique is illustrated by measurement of the intrinsic abrasion resistance of thin PVD coatings of TiZrN, ZrNbN and TiNbN on both flat and cylindrical tool steel and flat stainless steel substrates. The ability to measure the wear resistance of both a coating and its substrate, independently of each other and by a single test, is confirmed by experiment.

Development of the erosion durability technique for thin coatings

Abstract A novel method has been developed to assess the durability of thin coatings exposed to solid particle erosion. In this test a stream of particles travels from a parallel nozzle to strike a coated surface. The divergence of the stream leaving the nozzle can be described analytically and the flux of particles at any point may thus be predicted. By monitoring the growth in the diameter of the wear scar the critical dose Q c of particles required to remove the coating may be determined and used to describe the durability of the coating. The behaviour of a series of well-characterised titanium nitride PVD coatings on tool steel substrates has been studied. Q c depended on the erosion conditions (particle type, shape, size and velocity) and on the thickness of the coating. The effects of these parameters were systematically investigated and an analytical relationship determined which allows values of durability to be normalised. The work has provided an improved understanding of the properties determined by the erosion durability test and has also suggested methods for tailoring the technique to investigate different aspects of coating performance such as adhesion.

Mechanical properties of amorphous Ta100 − xCrx thin films

Abstract The mechanical properties—hardness, Youngs modulus and wear resistance—of thin, amorphous films of Ta 100 − x Cr x were studied by nano-indentation and micro-abrasive wear tests. The films, deposited with two independent magnetrons, had thicknesses between 1.8 and 2.2 μm. With compositions varied in small steps from x = 28 to x = 66, no influence on the mechanical properties of chemical short-range order, which might be expected close to compositions where crystalline compounds form, was observed. The correlation between microscale wear resistance and indentation hardness is discussed for these alloys as well as for TiAl metallic glasses.