H.M Hawthorne

Comparison of slurry and dry erosion behaviour of some HVOF thermal sprayed coatings

The performance of 10 high velocity oxy-fuel (HVOF) sprayed coatings has been evaluated under both dry particle and slurry erosion conditions at 90° and 20° impingement angles. The coatings were six WC cermets with Co- or Ni-based matrices, a Cr3C2–NiCr composite and three metallic alloys. Slurry jet erosion experiments were carried out using a 9 wt.% alumina/water slurry with 35 and 200 μm angular alumina particles, at an impingement velocity of 15 m/s and flow rate of 18 l/min in a re-circulating loop apparatus. Dry erosion was effected in a once-through mode with 50 μm diameter alumina particles at a velocity of 84 m/s and feed rate of 2 g/min. Dry erosion rates three orders of magnitude greater than slurry erosion rates were due mainly to the differences in actual particle-target impact velocities. Coating composition, microstructural integrity and hardness were the major determinants of relative erosion resistance in all tests, with matrix corrosion resistance influencing the surface damage resistance of cermets in longer duration aqueous slurry tests.

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.

Correlations between microstructural parameters, micromechanical properties and wear resistance of plasma sprayed ceramic coatings

Abstract The micromechanical integrity of a ceramic plasma sprayed (PS) coating is determined by the size and distribution of the defects found in the coating, such as porosity, the inter-lamellar microcrack density, the intra-lamellar microcrack density as well as the lamellar, or splat, dimensions. In this work, several micromechanical tests were used to advance our understanding of the relationships between the different microstructural parameters found in PS ceramic coatings. The tests included depth sensing indentation, micro and macrohardness testing, and controlled scratch testing. Abrasive and erosive wear tests were performed on the same set of coatings, including plasma sprayed alumina and chromia, as well as sintered alumina as a reference material. The best correlations were found between the material hardness ( H ), the level of porosity ( P ) and the abrasive wear volume ( W ). Knoop hardness measurements provided the best correlation with wear data, followed by scratch hardness and Vickers hardness. An exponential function of the type W = k / H n was found, where k and n are constants. A similar function describes the correlation of wear volume with the elastic modulus of the coating. Fracture toughness could only be correlated with wear volume when combined with hardness in a function of the type W = k / H 0.5 K c 0.5 . The incorporation into this function of a “microstructural factor” M = P n improves the correlation.

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.

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.

Measuring the adhesion of sol-gel derived coatings to a ductile substrate by an indentation-based method

Abstract The adhesion of sol–gel derived coatings to underlying substrates is a critical parameter. However, measuring the adhesion can be very difficult because such coatings have low cohesive strength and are brittle. In this study, an indentation-based method was used to measure the adhesion of sol–gel derived coatings to ductile substrates. A diamond Rockwell C indenter was driven through a sol–gel coating and into the underlying substrate. Extensive plastic deformation of the substrate caused an annular crack to develop at the coating–substrate interface. A microindentation technique was developed to measure the radius of the annular crack. The interface toughness was then calculated from the crack radius value, along with coating thickness, residual stress and mechanical properties of the coating and substrate.

A study of single particle–target surface interactions along a specimen in the Coriolis slurry erosion tester

Abstract The Coriolis test is used for evaluating the slurry erosion behaviour of materials. Experiments with a model single glass bead-in-water “slurry” have revealed the nature of, and extent of damage at, all contact sites along a soft copper specimen surface. The results confirm previous theoretical predictions that in the Coriolis test mode erodent particles interact with the specimen in a series of low angle impacts of decreasing angle, rebound height and normal velocity component along the specimen.

A controlled scratch test for measuring the elastic property, yield stress and contact stress–strain relationship of a surface

Abstract The procedure for a controlled scratch test to measure the elastic property, yield stress and contact stress–strain relationship of a surface is described. A hard sphere scratches the surface at a progressively increasing load. The contact deformation is firstly elastic and then elastic–plastic. The contact width between the sphere and the surface as a function of normal load is measured and the relationship between the contact pressure and scratch strain is obtained. The elastic property and yield stress of the surface are then derived from the curve with good accuracy. The strain hardening ability of the surface can also be estimated from the curve. This method is very valuable for thick coatings and brittle materials in which these mechanical properties are difficult to obtain by other methods.