Yongsong Xie

Effect of contact geometry on the failure modes of thin coatings in the scratch adhesion test

To use the scratch adhesion test for assessing the adhesion of a hard, thin coating to its substrate, it must be ensured that the failure event represents the loss of adhesion, since many coatings fail by non-adhesive modes in this test. This study demonstrates, theoretically and experimentally, that indenter-induced bending stress is very likely to cause cohesive failures, thus, the adhesion of a coating to its substrate cannot be assessed in many scratch adhesion tests. To induce adhesive failure but suppress cohesive failure in the scratch adhesion test, the compressive coating stress should be high but the bending-induced stress should be low. This can be achieved by using an indenter with large radius and a high normal load. To assess coatings with good adhesion to the substrate, the radius of the most commonly used Rockwell C indenter may not be large enough.

Wear mechanism of plasma-sprayed alumina coating in sliding contacts with harder asperities

The wear mechanism of plasma-sprayed alumina was studied by scratching a model alumina coating using a diamond indenter. Single scratching on polished virgin surface, repeated scratching over the same track and parallel interacting scratching were investigated. The predominant mechanism of material removal was found to be microfracture within the quasi-plastic zone, which takes place preferentially at splat boundaries and pre-existing cracks and is driven by inelastic strain. When scratching was made on a virgin surface the wear rate was relatively low, but when parallel scratches interacted, subsequent scratching was made on already heavily strained material, thus, the wear rate was much higher. The wear debris produced during single and parallel interacting scratching was in the form of small angular particles whose dimensions were related to the splat structure and pre-existing cracks. In repeated scratching over the same track, surface material experienced repeated inelastic deformation and, thus, crack development at the splat boundaries, and this resulted in the production of thin platelet wear debris.

The damage mechanisms of several plasma-sprayed ceramic coatings in controlled scratching

Abstract The damage mechanisms of several widely used plasma-sprayed ceramic coatings were studied by single point scratch tests using diamond indenters. The ceramic coatings investigated were Al2O3, Al2O3/13% TiO2 and Cr2O3. Single scratching on polished virgin surface, repeated scratching over the same track and parallel interacting scratching were investigated. Microstructures of the coatings, their surface and subsurface damage patterns due to the scratching, and the morphology of wear debris were examined. It was found that the unique microstructure of the coatings determined their wear and related damage mechanisms. The predominant mechanism of material removal in the scratching of Al2O3 and Al2O3/13% TiO2 coatings was microfracture within the plastic region, which takes place preferentially at pre-existing microcracks and micropores and is driven by plastic strain. The mechanism of material removal in the scratching of the Cr2O3 coating depended on contact load and contact geometry. When the contact load was less than a critical value that was related to contact geometry, the predominant mechanism of material removal was the same as that of Al2O3 and Al2O3/13% TiO2 coatings. When the contact load exceeded the critical value, the predominant mechanism of material removal was macro-fracture controlled by lateral cracking.

Wear rates and specific energies of some ceramic, cermet and metallic coatings determined in the Coriolis erosion tester

Abstract A series of ceramic, cermet and metallic coatings of known resistance to slurry jet erosion has been subjected to sliding bed wear using a 10 wt.% aqueous slurry of 80 grit crushed alumina particles in the Coriolis erosion tester. Wear scar cross-sections have been measured and specific energies for material removal calculated for these coatings as well as for some bulk solid materials for reference purposes. Thermal-sprayed coatings included high velocity oxygen-fuel (HVOF)-sprayed WC–12Co, Cr3C2/NiCr and 316L steel, plasma-sprayed (PS) Al2O3 and Al2O3/TiO2 and arc-sprayed 440C steel. Pulsed laser clad coatings of two FeCrB based alloys were also tested. The Coriolis erosion tester consists of a 150 mm diameter rotor with a diametral passage in which a flat plate specimen is located on each side, equidistant from the rotation centre. Slurry, delivered to a central orifice, is constrained to flow outwards through the channel as the rotor is turned at speeds up to 7000 rpm, the erodent particles being pressed onto the test specimen surface by the Coriolis force. The cross-sectional area of the scar is measured as a function of distance from the rotation centre and, through a summation of an expression for frictional work done, a value for the specific energy for material removal (essentially a measure of erosion resistance) is calculated. The test method seeks to simulate the wear environment in slurry pumps and cyclones in which particles move rapidly over the test surface either singly or in a bed. The wear rate of a WC–Co cermet was found to be ∼1000 times lower than that of 316L stainless steel. The remarkable ability of the Coriolis erosion tester to discriminate in the erosion performance of different materials is discussed in terms of particle trajectories and the probable wear mechanism. Coriolis test results are compared with those for slurry jet erosion and the applicability of the two tests for erosion resistance ranking of hard coating materials is discussed.

On the possibility of evaluating the resistance of materials to wear by ploughing using a scratch method

Abstract Although the plastic strain that a material can withstand in sliding contact is an important mechanical characteristic determining its resistance to ploughing wear, this material property could not be measured so far. In this study, a scratch test to measure the critical plastic strain is explored. A hard spherical indenter scratches the tested surface under progressive loading to induce an increasing plastic strain on the surface. When the plastic strain exceeds the limit of plastic deformation on the surface, micro-fracture takes place at the ridges of the scratch groove. The critical load to initiate this micro-fracture is detected in the scratch test and then the critical plastic strain is calculated. A measure of the resistance of a material to wear by ploughing, this critical plastic strain to micro-fracture has been used both for evaluating the cohesion of coatings and for screening wear-resistant materials.

A model for compressive coating stresses in the scratch adhesion test

Abstract Despite the widespread use of the scratch adhesion test, there is no model available to determine the critical coating stress for initiating interfacial failure, so that ranking the coating–substrate adhesion of coated systems with different mechanical properties is impossible. In this study, a mathematical model is developed to calculate the distribution of compressive stresses in a thin coating induced by a scratch indenter. For ease of use in practice, a simple equation, σ Sm =0.15 ( P c H f / H ) 0.5 E f 0.3 E 0.2 / R , is then derived from the model, where σ Sm is the critical mean compressive stress in the coating for interfacial failure, P c is the critical normal load measured from the scratch adhesion test, H f and E f are the hardness and Youngs modulus of the coating, H and E are the hardness and Youngs modulus of the substrate, and R is the indenter radius. This equation is useful for ranking the coating–substrate adhesion of different coated systems, or, for estimating the critical mean coating stress for interfacial failure.

Characterization of chemically bonded composite sol–gel based alumina coatings on steel substrates

Abstract Phosphate-bonded sol–gel composite alumina coatings were prepared on stainless steel substrates at processing temperatures of 300, 400 and 500°C. Mechanical property and electrochemical characterisation has been carried out and relationships sought between both processing temperature and coating microstructure. Coating corrosion and electrochemical behaviour is largely controlled by the degree of cracking and porosity in the coatings, which is minimum in those processed at the lowest temperature. Little correlation was found between coating processing or microstructure and coating mechanical properties (micro and scratch hardness, elastic modulus) or residual stresses but the interface toughness, as measured by a high load indentation method, increased with process temperature on coatings deposited on sand-blasted substrate surfaces.

Modelling slurry particle dynamics in the Coriolis erosion tester

Abstract The Coriolis erosion tester consists of a rotor with a diametrical passage in which two flat specimens are located equidistant from the centre. At high rotation speeds, slurry flows outwards from the centre by centrifugal force while the erodent particles are directed towards the specimen surfaces by the Coriolis force. This test mode offers a quick, simple and reproducible means of simulating the wear environment in slurry pumps and cyclones in which particles move rapidly over component surfaces either singly or in a bed, and clearly distinguishes between the erosion performance of different materials. A model is presented to describe particle trajectories and particle impact velocities quantitatively for both dilute slurries and those with relatively high solids concentrations flowing through the Coriolis erosion tester. Experiments using either angular alumina particles or glass beads suspended in water and a range of both coatings and solid specimens have provided information on damage modes and erosion resistance which shows that the predominant particle–test surface interaction in the Coriolis erosion tester is low-angle impact. The influence of rotational speed, particle size, particle density and concentration on impact angle and impact velocity are predicted by the model and these are consistent with observed wear patterns. The erosion resistance of a material depends critically on its elastic deformation, plastic deformation and fracture properties, and this material response is interpreted in terms of contact mechanics.

A new Coriolis slurry erosion tester design for improved slurry dynamics

Abstract The Coriolis slurry tester provides a means of discriminating between the resistance of different materials to slurry erosion damage under low interaction intensity (low impingement angle and low normal velocity impact) conditions. A new design of the rotor–specimen holder assembly has been evolved to provide better control of the slurry flow over and along test specimen surfaces. Wear patterns on tester components reveal that slurry flow is similar to that predicted theoretically. The effects that the less turbulent fluid flow has on the response of specimen materials to slurry erosion, as measured either by specific energy or erosion resistance parameters, are also discussed. The new tester overcomes the sensitivity of earlier designs to small variations in the dimensions of specimen holders, or specimen positioning inserts, consequent upon machining tolerances.

Thin Film Solid Oxide Fuel Cells Deposited by Spray Pyrolysis

Two techniques of spray pyrolysis, namely, electrostatic and pneumatic spray deposition, were used to deposit samaria-doped ceria (SDC) electrolyte and lanthanum strontium cobalt ferrite (LSCF) cathode on cermet or metal supported anodes for solid oxide fuel cells (SOFCs) operated at reduced temperature. The deposition processes, the properties of the deposited films, and the electrochemical performances of the fabricated cells are reported in this paper. The deposited SDC electrolytes were dense and gas-tight, and had good adhesion to the underlying anodes. The deposited LSCF cathode had a preferred morphology to facilitate the transport of oxygen gas and effective contact with the electrolyte. Button cell testing indicated that the SOFCs with electrolyte or cathode deposited by spray pyrolysis had good electrochemical performance. This study demonstrated that spray pyrolysis is a cost-effective process for fabricating thin film SOFCs, especially metal supported SOFCs.