D.S. Rickerby

The use of scratch adhesion testing for the determination of interfacial adhesion: The importance of frictional drag☆

For many years, despite a basic lack of understanding of the mechanics on which the test is based, the scratch adhesion test has provided a simple, rapid means of assessing the adherence of thin, hard wear-resistant coatings such as titanium nitride. Recently, some progress has been made on the modelling of the scratch adhesion test and these previously published models are critically reviewed. The test can be considered as a combination of (i) an indentation stress field, (ii) an internal stress field and (iii) a frictional stress field. The importance of each of these in determining the levels of adhesion for a number of titanium nitride/substrate systems will be discussed. In particular, the results obtained from changes in diamond stylus-coating (surface) interfacial friction (a frictional drag term) are presented and the apparent changes in coating-substrate adhesion (critical load Lc) which result are discussed.

Advanced surface coatings : a handbook of surface engineering

Advanced surface coatings: a handbook of surface engineering , Advanced surface coatings: a handbook of surface engineering , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

The role of titanium in the abrasive wear resistance of physically vapour-deposited TiN

Abstract The use of physically vapour-deposited TiN coatings for tribological applications is now commonplace, though the reasons for the improvements in wear behaviour they produce are not fully understood. In order to promote good adhesion between substrate and coating a thin titanium interlayer is often used. The presence of this interlayer in the case of sputtered coatings, or the presence of any unreacted titanium in the case of arcevaporated coatings, can have significant effects on the abrasive wear resistance of a coated component. Abrasive wear tests have been performed on sputtered TiN coatings on stainless steel with a range of titanium interlayer thicknesses. Best wear properties are obtained for the thickest interlayer due to the increased interface toughness in this system. Arc-evaporated coatings perform better than similar sputtered coatings under mild abrasive conditions, but the behaviour is reversed for more severe abrasive conditions. A model is proposed to account for these observations, but clearly the presence of titanium can have both advantages and disadvantages for the production of abrasion resistant coatings by physical vapour decompositon techniques.

COMPOSITIONAL, MICROSTRUCTURAL AND MORPHOLOGICAL EFFECTS ON THE MECHANICAL AND TRIBOLOGICAL PROPERTIES OF CHROMIUM NITROGEN FILMS

Abstract It is now well established that the microstructure of physically vapour deposited films dictates many of their mechanical properties which in turn determine the tribological performance of the film. In particular phase composition and texture need to be taken into consideration if the properties of a film are to be fully understood. By controlling the partial pressure of nitrogen during reactive sputtering it has been possible to produce films of compositions ranging from pure chromium to CrN. At low nitrogen partial pressures the films contained a mixture of Cr2N and CrN phases, but with increasing nitrogen partial pressure the CrN phase became dominant and exhibited a change in texture from {200} to {111}. The hardness, adhesion and tribological behaviour of these films (under both abrasive and adhesive wear conditions) have been determined and related to microstructure and phase composition. Under similar deposition conditions of pressure, coating time and substrate bias voltage, the hardest films consist of {200}-textured CrN, an effect attributed to the lower density of the films containing a mixture of Cr2N-CrN. Best abrasive wear properties are obtained for the hardest films, but the results of sphere-on-disc wear tests show that optimum sliding wear behaviour occurs for the smoothest of the {200}-textured CrN films.

The abrasive wear resistance of sputter ion plated titanium nitride coatings

Abstract Previous work has shown that under abrasive wear conditions it is not sufficient to consider coating hardness in isolation when seeking a guide to coating selection; it is more meaningful to talk in terms of the load-bearing capacity of the coating-substrate system. For titanium nitride coatings deposited onto austenitic stainless steel the variation in resistance to abrasive wear has been evaluated as a function of substrate bias voltage and the results have been compared with these earlier studies. Substrate bias has been identified as a particularly important systems parameter, since it allows some stress relaxation to occur within the coating because of its effect on porosity levels; this has important consequences with regard to adhesion, wear resistance and film hardness. With low substrate bias voltages, coatings degrade by a microchipping mechanism since fracture occurs relatively more easily in such open columnar microstructures, which are further characterized by low levels of internal stress accompanied by a poor load-bearing capacity. With increasing substrate bias much denser coatings are produced which result, initially, in better resistance to abrasive wear through increased load support. However, these improvements in coating microstructure result in higher levels of internal stress and these, when taken along with a reduction in the scratch adhesion critical load for failure Lc, eventually lead to a decrease in abrasive wear resistance with increasing bias voltage. The mechanism(s) by which the coatings degrade are described as a function of substrate bias, and the results are correlated with measurements of hardness, internal stress and critical load for coating detachment.

Engineering with surface coatings: The role of coating microstructure

Abstract There is now great interest in the tailoring of the surface properties of a bulk material by the application of surface coatings, with a view to imparting to that surface specific characteristics which cannot be attained in the bulk material. If the maximum benefits are to be realised for a particular coating-substrate system, it is of fundamental importance that the interrelationship between the coating microstructure and its properties are understood. By reference to work on physical vapour deposited coatings such interrelationships are explored, and the importance of the coating microstructure in dictating many of the physical properties of the coating, for example hardness, adhesion, levels of internal stress, composition etc., are highlighted.

Multi-pass scratch testing as a model for abrasive wear

Abstract The single-pass scratch test is often used to give information about adhesion of coatings to the substrate. At the critical load ( L c ) the coating becomes detached from the substrate and can be removed by chipping. However, the particle loading in real tribological systems is much smaller than at the critical load in the single-pass test and, although failure by detachment can still occur, in this case several passes over the same scratch are necessary before sufficient cumulative damage occurs which can then result in coating failure. Multiple-pass tests have been performed along the same scratch track at a range of loads below L c and acoustic emissiion monitored to detect the onset of coating failure by both detachment and cohesive cracking. The number of passes necessary to cause failure increases as the normal load is decreased, and results are presented for a number of coating/substrate systems and these are related to the known tribological behaviour of the coatings.

The sliding wear of titanium nitride coatings

Abstract Although the mechanisms by which titanium nitride (TiN) coatings can affect the abrasive wear resistance of coated components are well documented, relatively little is known about the sliding wear behaviour of these materials. Sphere-on-disc tests have been performed on sputtered TiN coatings, deposited at a range of bias voltages, using both coated and uncoated spheres. The amount of sphere and disc wear decreases with substrate bias, though the wear rate for the disc increases at the very highest bias voltages. The coefficient of sliding friction is approximately constant except for the case where coated spheres slide on coated flats. The mechanisms of adhesive wear are based on the local transfer of iron, which subsequently becomes trapped in the open regions within the coating microstructure, increasing the sphere-coating adhesion at these positions. Consequently, best sliding wear properties result from hard, dense coatings as produced at high substrate bias voltages, although oxidative wear becomes increasingly important as the bias is increased.

Erosive wear of thermally sprayed tungsten coatings

A detailed study has been made of the erosion behaviour of air plasma sprayed (APS) and vacuum plasma sprayed (VPS) tungsten coatings. The erosion rates observed under a variety of test conditions are discussed in terms of wear mechanisms, coating microstructure and mechanical properties. Both coating systems exhibited similar trends in wear behaviour as a function of impingement angle, particle velocity and size. However, the steady-state erosion rates of APS coatings were an order of magnitude greater than those of either VPS coatings or bulk material tested under similar conditions. For APS tungsten coatings, the higher erosion rates are attributed to the loss of individual splats by brittle boundary fracture which is caused by the presence of an oxide layer at the intersplat boundaries. In contrast, VPS tungsten coatings were well bonded and oxide free and the material removal mechanisms involved plastic deformation, i.e. ploughing of the surface of splats. Scratch adhesion and tensile test measurements were performed to quantify the differences in the mechanical properties of the coatings in terms of bulk modulus and intersplat cohesion.

The Hardness and Elastic Modulus of TiN Films as Determined by Ultra-Low Load Indentation

The mechanical properties of a series of titanium nitride films on stainless steel substrates have been evaluated using an indentation technique with a mechanical properties microprobe (MPM). The MPM makes possible measurement of film properties without contribution by the substrate material. The titanium nitride films were deposited with a PVD technique known as sputter ion plating (SIP). Deposition substrate bias was varied from 0 to {minus}120 V, while keeping other deposition parameters constant. With increasing negative substrate bias, dramatic increases in hardness and elastic modulus have been observed. 2 refs., 2 figs.