Application of thin layer activation to measuring tool wear in metal cutting

Thin layer activation (tla) as a method for measuring tool wear is discussed in the context of previously available methods and shown to be more accurate, quicker and less costly. The concept of tool wear resistance, a function of cutting time and wear, obtained experimentally by tla, is introduced and shown to be a useful aid to determining tool life     

A study on wear resistance of the white layer

The white layer has received some consideration, but there are two different opinions on its wear characteristics. It is considered to be of tribological advantage in some instances. However, the microcracks and voids formed by adiabatic shearing are precursors to fracture and should decrease the wear resistance. As to the white layer previously generated by impact wear, the wear characteristic of the white layer is studied by a pin on disc machine in the present paper. It is found that the white layer causes delamination of material, which increases the wear loss significantly. The delamination of the white layer, being prior to the cutting wear, is the primary wear mechanism for materials with the white layer.     

A theoretical model for the prediction of initial growth angles and sites of fretting fatigue cracks

The crack initiation mechanisms under fretting fatigue conditions are investigated. Fretting fatigue tests have been conducted on an aluminium alloy. Two major crack initiation mechanisms are experimentally observed. They are theoretically identified by employing a simple dislocation dipole model and taking into consideration the reverse sliding along dislocation layers and the influence of the distance between these layers. It is shown that cracks may initiate either by an extrusion-intrusion mechanism or a fatigue tensile process. Two parameters respectively linked to each of these initiation mechanisms are proposed to predict crack location. The theoretical crack initial growth directions and locations are shown to correlate very well with the experimental ones. With regards to these initiation processes, a spall detachment mechanism is proposed considering the interaction of microcracks.     

Environmental effects on the formation process of adhesive wear particles

Three kinds of experiments have been conducted to study the effect of environmental molecules on the formation process of adhesive wear particles. First, the growth process of transfer particles was continuously observed by scanning electron microscope. It was understood that the growth of the transfer particles was due to the continuous conversion of the disc surface material into transfer particles. Next, wear tests between a Sn pin and Sn disc were conducted in an oxygen environment from 7 × 10⁻³ Pa to 1.0 × 10⁵ Pa. The size and hardness of wear particles and the wear volume increased with increasing oxygen pressure. Wear tests between Sn and Sn were also conducted while Au metals were sputter-deposited onto the friction surface. The deposition of Au had the same effect on the size and hardness of the wear particles as the oxygen atmosphere. In general, the hardness of metals is increased by the small amount of impurity atoms in them. The inclusion of oxygen or gold atoms on the surface and in the transfer particle is supposed to have the same effect of enhancing the shear strength of the interface and the transfer particles. This is the possible mechanism of the continuous shear fracture within the bulk which results in the growth of transfer particles.     

Comments on the sliding wear of metals

The author reviews selected experimental results which have contributed to improved understanding of sliding wear processes. The emphasis is on the chemical and structural changes which occur at and near the surface of metallic materials during sliding in different environments. The importance of plastic deformation, fracture, transfer, mechanical mixing, phase transformations and oxidation is discussed. Examples of transitions are described, and interesting correlations noted. In selecting the content of this paper, the author includes controversial results and conclusions and raises questions about the development of wear equations, interpretations of the wear coefficient, the importance of adhesion, the roles of hardness, the causes of transitions and the location of debris-producing cracks.     

Strain rate response and wear of metals

Models and mechanisms proposed for wear of metals are reviewed. It has been observed earlier that in severe wear if the microstructure, which evolves in the subsurface in response to the traction, is deleterious and unstable, cracks may nucleate at the site of these instabilities leading to the generation of debris. It is postulated here that, given the traction, metals respond to the local strain rate and temperature by evolving a microstructure in a unique fashion. If the response is deleterious a crack is nucleated. Further, if there is sufficient energy available locally the crack is propagated to generate debris particle. The strain rate responses of titanium and copper are recorded in compression and used to interpret the wear characteristics of these metals in the sliding speed range of 0.1 to 4 ms⁻¹.     

Friction and wear in silicon nitride-steel and cemented carbide-steel pairs in lubricated sliding

Friction and wear tests between a stationary block and a rotating ring under high contact pressure of about 200 MPa were carried out at room temperature under lubrication with a light mineral oil at a sliding distance of 500 m. The block was silicon nitride and cemented carbide, and the ring was bearing steel. The effect of phosphorus and sulphur contained in the mineral oil on the friction, the roughness of the worn surface and the wear of the steel ring is discussed in relation to both pairs. Sulphur was effective in reducing the coefficient of friction of the cemented carbide block-steel ring pair, while phosphorus was successful in decreasing the wear of the steel ring paired with the silicon nitride block. The surface analysis of the steel ring using X-ray photoelectron spectroscopy (XPS) shows that the peak intensities of sulphur or phosphorus beneath the surface depend upon the material of the counterpart, silicon nitride or cemented carbide blocks.     

The effect of slip amplitude and test time on fretting wear in metal-metal contact

A study was conducted to determine the effect of amplitude and test time on surface damage in metal-to-metal contact under lubricated conditions. The test set up consisted of a ball loaded against a flat disc, with an external drive imparting a linear oscillatory motion to the ball on the flat. The materials were steel AISI 52100/AISI 52100, and the lubricant was ISO VG 220. Damage characteristics were defined for amplitudes in the range of 5 to 50 microns and for test times of 10 to 360 min.     

Multilayer composite ceramicmetal-DLC coatings for sliding wear applications

The design of anti-friction coatings able to perform well in different wear conditions without lubricants requires a combination of adequate hardness and toughness, good adhesion, a low friction coefficient and a low wear rate. Recently introduced metaldiamond like carbon (DLC) coatings produced by magnetron sputtering of metals from targets, which are to a controlled extent covered with carbon from the chamber atmosphere, can be a step towards the achievement of such a combination. These coatings consist of an amorphous a:CH matrix with the possible incorporation of metal (Ta, W, Nb, Ti), metal carbide and/or graphite grains. Previous studies of Ti_x%-DLC coatings showed their good protective properties against abrasive, impact and single scratch wear, as well as a requirement for supporting interlayers to successfully apply such coatings to low-cost steels. In the present work an example of the selection of metal-ceramic Ti-TiN-TiCN supporting interlayers is given based on studies of their morphology, structure and mechanical properties. This resulted in the development of Ti-TiN-TiCN-[TiC-(Ti_x%-DLC)] multilayer composite coatings. Several coatings were prepared with the same supporting interlayer and a variation in the preparation of the Ti_x%-DLC layer. Ball-on-disc experiments were carried out to investigate these coatings in conditions of sliding wear against steel and cemented tungsten carbide balls. CrN, TiN and TiCN coatings were also deposited and tested in the same conditions to provide a reference. Low friction coefficients (below 0.2 at an air humidity of 50% RH) in combination with low normalized wear rates were found for multilayer coatings with upper Ti_20%-DLC and Ti_35%-DLC layers.     

Effects of lubricants on friction and wear of Ti(CN)/1045 steel sliding pairs

The friction and wear properties of Ti(CN)/1045 steel rubbing pairs were investigated under dry and lubricated conditions by using a pin-on-disk tribometer. The selected speed range was 0.8 to 3.2 m/s and the load range was 58.8 to 235.2 N. Distilled water and a mineral oil (no additives) were used for lubrication, respectively. The wear of Ti(CN) ceramic under dry conditions was caused mainly by adhesion between the rubbing surfaces and the microfracture of Ti(CN). With the load and speed increasing, the adhesion and diffusion between rubbing surfaces increased and resulted in wear increasement of Ti(CN). Because of the brittleness of ceramics, the microfracture wear of Ti(CN) increased rapidly when the load was raised to some high values. The lubricating and cooling effects of the lubricants could improve the frict on and wear. Compared with water, oil was much better in improving the tribological properties. The analysis results obtained from XPS and AES examinations showed that ferrous oxide was produced on the wear scars, which could reduce the adhesion between the rubbing surfaces to some extent. The lubricating effects of the oil under boundary lubrication conditions were attributed to the formation of carbon films on the rubbing surfaces.     

A review of the performance of engineering materials under prevalent tribological and wear situations in South African industries

The wear of materials used on machinery operating in a wide range of industrial situations such as mining, energy production and agriculture can cause serious inefficiencies, sudden breakdowns and consequential financial losses. Our work over two decades has concerned industrial problems encountered in South African industry, and laboratory simulations of abrasion, abrasion plus corrosion, adhesive wear, impact wear, cavitation, solid particle erosion and erosion plus corrosion situations have been successfully undertaken. Materials investigated include plain carbon and alloy steel, stainless steels, aluminium alloys, cast irons, tungsten carbide cermets, ceramics, polymers, composites and various surface treated and coated materials. The paper reviews the investigations carried out in our laboratories and conclusions we have made.     

A comparative study on the microstructure and tribological properties of Si3N4 and TiN films produced by the IBED method

The tribological properties of Si₃N₄ and TiN thin films produced by ion beam enhanced deposition (IBED) were compared on a SRV friction and wear testing machine. The friction coefficient of all thin films shows a descending tendency with increase in load, and is lower than that of 52100 steel. All the IBED films show a much better wear resistance than 52100 steel, especially in the higher load and frequency ranges; it can reach six times that of the latter. In order to understand the reasons for their excellent properties, the microstructure, microhardness and bonding strength with the substrate were analysed by SEM, X-ray diffraction, Knoop hardness and scratching test methods separately. The results show that the TiN(1) film exhibits the best tribological properties, which are closely related with its greater hardness and bonding strength.     

Materials transfer and formation of mechanically mixed layer in dry sliding wear of metal matrix composites against steel

The wear behavior of a composite of a pure Al matrix reinforced with 20 vol% SiC particles against quenched 1045 steel was investigated. Material was found to transfer from the counterface to the wear surface of the composite, where a mechanically mixed layer (MML) was produced. A new mechanism, named the debris burial mechanism, for the formation of the mechanically mixed layer (MML) is proposed and discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Amorphous hydrogenated carbon films for tribological applications II. Films deposited on aluminium alloys and steel

This paper describes the methods for the deposition of AHC films on aluminium alloys (2024, 7075 and an additional Al-Si alloy) and AISI 4340 steel. Both unmodified and silicon modified AHC films were deposited. AHC films could be deposited on aluminium alloys without any interlayer. The deposition of AHC films on steel required an interlayer which could be aluminium, silicon or chromium. Thin films (1–2 μm) deposited on aluminium alloys and steel influenced durability of films and friction coefficients in contact with steel. These were believed to be due to plastic deformation of substrates. Deposition of a thicker coating system (interlayer + AHC) reduced friction coefficients and also improved film durability. The durability of films deposited on steel substrates was evaluated under both unlubricated and lubricated conditions for 5.5 million cycles under 4.4 N load and up to 2.5 m/s sliding speed. Although there was wear, the films survived 5.5 million test cycles under unlubricated sliding, but in the presence of two lubricants, the film wear was very small and could not be measured. It was observed that the wear of the steel counterface in contact with silicon-containing AHC films could be higher than that against an uncoated steel in the presence of certain lubricants.     

Comparison of the fretting wear of 100Cr6/100Cr6, Si3N4/Si3N4 and Si3N4/100Cr6 contacts in lubricated and dry conditions

The fretting wear behaviour of bearing steel against bearing steel, silicon nitride against silicon nitride, and silicon nitride against bearing steel, was investigated under lubricated and dry conditions. Amplitudes in the intermediate 5 to 50 μm range, and test durations from 10 to 360 min, were studied. Light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) were employed to determine the detailed nature of the friction and wear processes. In the silicon nitride against silicon nitride contact, brittle fracture of Si₃N₄ grains, and tribochemical reaction creating an amorphous layer on the mechanically damaged surface, were found. The main mechanism of fretting wear in the case of bearing steel against bearing steel contact was delamination. In the silicon nitride against bearing steel contact, chemical reactions predominated.     

The friction and wear properties of nanometre SiO2 filled polyetheretherketone

Nanometre SiO₂ filled-polyetheretherketone (PEEK) composite blocks with different filler proportions were prepared by compression moulding. Their friction and wear properties were investigated on a block-on-ring machine by running a plain carbon steel (AISI 1045 steel) ring against the composite block. The morphologies of the wear traces and the transfer film were observed by scanning electron microscopy (SEM). It was found that nanometre SiO₂ filled-PEEK exhibited considerably lower friction coefficient and wear rate in comparison with pure PEEK. The lowest wear rate was obtained with the composite containing 7.5 wt.% SiO₂. The SEM pictures of the wear traces indicated that with the frictional couple of carbon steel ring/composite block (fillec with 7.5 wt.% filler), a thin, uniform, and tenacious transfer film was formed on the ring surface. It was inferred that the transfer film contributed largely to the decreased friction coefficient and wear rate of the filled PEEK composites.     

The lubricated wear of ceramics : Part II: The wear and friction of silicon nitride and steel in the presence of mineral oil or an ester based lubricant

The friction and wear characteristics of combinations of silicon nitride, alumina and AISI 52100 steel in the presence of mineral oil containing anti-wear, dispersant and detergent additives have been investigated in a tri-pin-on-disc machine. The tests were carried out at a nominal temperature of 100°C for a range of sliding speeds, loads and total sliding distances. In Part II of this two-part paper a comparison will be made between the tribological performance of these sliding pairs of materials in mineral oil and ester based lubricant environments. The results of the investigation showed that the alumina performed relatively poorly under these test conditions, whereas silicon nitride showed good potential as an improved wear resisting material compared with 52100 steel. Wear factors of the order of 10⁻¹⁰ mm³/Nm were deduced for the alumina, while values as low as 10⁻¹¹ mm³/Nm were typical of the silicon nitride sliding against 52100 steel discs. The alumina pins wore by a process of brittle fracture at the surface, whereas the silicon nitride pins wore primarily by a tribochemical polishing mechanism. The rate of tribo-chemical wear was found to be proportional to the nominal contact area.     

Influence of load and speed on wear characteristics of grey cast iron in dry sliding — selection for minimum wear

The wear of grey cast iron in dry sliding conditions has been studied with the aims of (1) finding the influence of working conditions on the wear rate, and (2) determining the region of speed and load where low wear is accomplished. Grey cast iron with flake and nodular graphite was submitted to investigation using a pin-on-disc machine. The results indicate that the flake graphite cast iron is more suitable for applications at speeds greater than 4 m s⁻¹ and lower loads, while nodular cast iron has greater wear resistance at lower speeds in the range of loads investigated - from 50 N cm⁻² to 200 N cm⁻²     

Dry sliding wear response of zinc-based alloy over a range of test speeds and loads: a comparative study with cast iron

This study pertains to the observations made during the sliding wear response of a zinc-based alloy in different test conditions. The effects of sliding speed and load on the wear behaviour of the alloy have been studied. The properties evaluated were wear rate, frictional heating and coefficient of friction. The wear performance of the zinc-based alloy has been compared with that of a conventional cast iron in identical test conditions. The wear rate of the samples increased with applied load and sliding speed while the seizure resistance (load) deteriorated with speed. The zinc-based alloy exhibited less wear rate and reduced frictional heating than that of the cast iron while friction coefficient followed a reverse trend. Observed wear response of the samples has been discussed in terms of specific features like lubricating, load carrying, microcracking and thermal stability of various microconstituents of the samples, and substantiated further through the features of wear surfaces, subsurface regions and debris.     

Friction and wear of Inconel 690 for steam generator tube in elevated temperature water under fretting condition

Recently, material of Inconel 690TT (thermal treatment) for the steam generator tubes in a nuclear power plant was substituted for the existing material of Inconel 600HTMA (high temperature mill-annealing). Inconel 690TT has more chromium than Inconel 600HTMA in order to improve the corrosion resistance. In this study, to evaluate the friction and wear characteristics of Inconel 690TT under fretting condition, the fretting tests as well as sliding tests were carried out in air and in elevated temperature water environment, respectively. Fretting tests of the cross-cylinder type were done under various applied normal loads, and sliding tests of pin-on-disk type were also carried out to compare with the results of the fretting test. In summary, the results of the fretting tests correlated with the results of the sliding tests. The wear mechanism of Inconel 690TT in air was delamination wear and the mechanism in water was affected by micro-pitting. Also, it was found that the fretting wear coefficients in water were increased with increase in the temperature of water.