D.W. Wheeler

Diamond coatings on tungsten carbide and their erosive wear properties

Abstract Diamond coatings up to ∼60-μm thick have been grown by microwave plasma CVD (MPCVD) on sintered tungsten carbide (WC) substrates, and their erosive wear properties are investigated under high velocity air–sand erosion testing. Two different sintered tungsten carbide (WC) substrates have been investigated and compared, the binder being either 6% Co or 5% Ni by weight. Significant differences in morphology, residual stress, adhesion and erosion performance are seen as a function of pre-deposition treatment, deposition conditions and the source of the substrates. Adherent coatings could be deposited to a thickness of ∼35 μm. They offer significantly better erosion resistance compared to uncoated substrates, with the erosion rate being lowered by up to a factor between ∼5 and 20 for particle test velocities of 148 and 63 m s−1, respectively. The steady-state erosion rates of the coatings are a function of a gradual micro-chipping mechanism. However, the life of the coating is dependent on the progression of sub-surface damage promoted by sub-surface shear stresses associated with the particle impacts. It is thought that the coating debonding is driven by the shear stresses interacting with the grain boundary porosity at the substrate/coating interface.

Design and performance of a high velocity air–sand jet impingement erosion facility

Abstract This paper describes the design, construction and capabilities of a high velocity air–sand erosion rig. It has been designed with the aid of computational fluid dynamics to approximately simulate the erosion conditions often experienced by subsea choke valves used in the offshore oil industry. It has also been designed to evaluate the erosion performance of CVD diamond coatings at sonic velocity. The rig is of the gas-blast design in which solid particles, typically sand 60–660 μm in size, are injected into a high velocity air stream and accelerated down a 16-mm-diameter tube, 1 m in length, before striking the sample under test. Tests can be carried out with particle velocities of up to 340 m/s under a wide range of sand fluxes, impact angles and standoff distances. The results of pressure, velocity and sand flux calibration work are described. In addition, preliminary experimental data on tests carried out on mild steel, bulk and sprayed tungsten carbide are also presented. The flexibility of the air–sand rig allows the erosion behaviour of materials to be studied under a wide range of conditions.

Erosive wear behaviour of thick chemical vapour deposited diamond coatings

This paper reports work carried out on 60–200 ?m thick CVD diamond coatings deposited on tungsten and cemented tungsten carbide substrates. The erosive wear behaviour of these coatings relative to cemented tungsten carbide is described. Erosion tests used quartz silica sand, on average 194 ?m in diameter, in air at a velocity of 268 ms?1. The erosion rates and micro-mechanisms, and their effect on coating life are presented as a function of coating thickness and the surface conditions of as-grown or lapped coatings. The eroded surfaces were studied by scanning electron microscopy (SEM) and surface profilometry. Ultrasonic imaging and taper polishing of tested samples were also performed to reveal sub-surface damage and to elucidate its contribution to coating degradation. The results suggest that the samples erode by a gradual chipping of grains in the early stages followed by the accumulation of damage at the coating–substrate interface. It is this latter feature which eventually leads to catastrophic failure of the coating along the interface. These features are discussed in the context of the classical erosion damage features normally exhibited by brittle materials as well as the coating microstructure. The propensity for coating debonding would suggest that improved coating adhesion would further enhance erosive wear behaviour of thick CVD diamond coatings.

Solid particle erosion of CVD diamond coatings

Abstract This paper reports solid particle erosion studies of 10–47 μm chemical vapour deposited (CVD) diamond coatings deposited on W and SiC substrates. Two erosion test facilities were used: a water–sand slurry rig and a high-velocity air–sand rig. The erodent used was silica sand with average diameters of 135 μm, 194 μm and 235 μm with the velocities in the range of 16–268 ms −1 and 90° nominal impingement angle. The erosion rates were plotted against particle kinetic energy and compared with those for cemented tungsten carbide and stainless steel. The samples were examined both pre- and post-test by scanning electron microscopy in order to determine the mechanisms of degradation suffered by the coating during the erosion process; the effects of thickness and microstructure were also examined. The time to failure of the coatings at 268 ms −1 was found to increase from 5 to 185 min over the range of coating thickness tested. The erosion mechanism at high-velocity conditions is thought to be a three-stage process consisting of micro-chipping, development of pin-holes and interfacial debonding, followed by catastrophic failure. However, it should be noted that, at particle velocities of 268 ms −1 , 46 μm CVD diamond coatings on tungsten displayed approximately six times the erosion resistance of cemented tungsten carbide.

Sand erosion performance of CVD boron carbide coated tungsten carbide

The erosion performance and the interaction between the micro-mechanisms of erosion and the microstructure of a chemical vapour deposited boron carbide coating are presented. Samples were tested using both water–sand slurry and air–sand jet impingements at 90° incidence. Tests used angular quartz sand with a mean diameter between 135 and 235 ?m and jet impingement velocities between 16 and 268 m s?1. The chemical vapour deposition (CVD) boron carbide coatings were 15–20 ?m thick and deposited on a range of substrates of sintered tungsten carbide with 6 to 15 wt.% metal binder. The results, relative to the erosion resistance of the uncoated substrates, show the coatings to have higher resistance (10 times) under lower energy impacts but similar resistance at higher energy impacts. The sintered boron carbide had a similar erosion resistance to that of sintered tungsten carbide except at high energy impacts where it outperforms tungsten carbide and CVD boron carbide by a factor of 2. The performance of these coatings against erodent mass and impact energy are discussed and related to the nano and micro brittle fracture mechanisms identified by detailed microscopy and predicted by Hertzian cone crack theory. Partial concentric spalling of the coating was also evident in regions where circular Hertzian surface cracks are present. These erosion mechanisms, primarily nano-chipping and crack propagation, are also related to the microstructure and composition identified by XRD analysis and Raman spectroscopy. These results, in conjunction with fracture toughness and micro-hardness measurements, suggest that the coating composition is not pure B13C2 but has less erosion resistant forms of boron carbide present such as B50C2

High velocity sand impact damage on CVD diamond

This paper describes a recent study of the damage mechanisms generated by high velocity-sand impact on diamond coatings deposited on tungsten substrates by chemical vapour deposition (CVD). The coatings were erosion tested using 90–355-?m diameter sand at a velocity of 268 m s?1 and the eroded coatings examined by scanning electron and acoustic microscopy. The images indicate that the circumferential cracks and pinholes are the main erosion features and are only located on debonded areas of the coating. This suggests that they could be formed by stress waves reflected from the coating–substrate interface, which interact with surface waves to generate circumferential cracks, the precursor to pinholes. The high spatial resolution of scanning acoustic microscopy enables the resolution of individual pinholes, thus, providing important evidence for identifying the mechanism responsible for the formation of circumferential cracks, the precursor to the pinholes. However, the acoustic images must be interpreted with care; in particular, it is important to compare microstructural features observed by acoustic microscopy with other techniques.

CVD Diamond: Erosion Resistant Hard Material

Abstract The present paper describes the erosion of 60 μ m thick diamond coatings on tungsten, 600 μ m thick free standing diamond brazed to tungsten carbide and two grades of cemented tungsten carbide and stainless steel type 316. The erosion rate of the diamond samples was between 12 and 30 times lower than that of tungsten carbide at a particle velocity of 268 m s-1. Moreover, the diamond coating displayed an erosion resistance approximately 2·4 times higher than the brazed diamond. However, pinholes were observed on the 60 μ m thick coating, which were not seen on the thicker diamond. Scanning acoustic microscopy showed these pinholes to be formed on regions of debonded coating and, therefore, they provide a visual indication of coating debonding, which eventually leads to the catastrophic failure of the coating. The mass loss associated with the formation of pinholes has been estimated to account for less than 5% of the total mass loss. Therefore, the coating life or time to failure tF rather than the volumetric erosion rate may be a more useful parameter by which the performance of different coatings can be compared.

Fracture of diamond coatings by high velocity sand erosion

This paper describes a study of the behaviour of diamond coatings when subjected to solid particle erosion from sand particles. The coatings were deposited by chemical vapour deposition (CVD) onto tungsten substrates and tested using a high velocity air–sand erosion test facility. The erosion tests were conducted using particle impact velocities of between 33 and 268 m/s. Examination of the eroded test specimens showed that the principal damage features were circumferential cracks and pin-holes. Comparison with Hertz impact theory revealed that the measured circumferential crack diameters were more than double the predicted Hertzian contact diameter. Moreover, a trend of increasing circumferential crack diameter with coating thickness, which is not predicted by Hertz, was found. Instead, the crack diameters showed good agreement with those predicted by the theory of stress wave reinforcement, which is more commonly associated with liquid impact damage of brittle materials. During impact, the bulk compression and shear waves are reflected at the rear surface of debonded regions of the coating to return to the front surface and reinforce the Rayleigh surface wave, which generates a tensile stress. Where this stress exceeds the local tensile strength of the coating, a ring of cracks surrounding the area of impact is created. The results from the present study therefore suggest that stress wave reflection is responsible for the formation of the cracks at locally debonded regions of the coating. This hypothesis was supported by images acquired using scanning acoustic microscopy, which showed that circumferential cracks and pin-holes were only found on areas of the coating that had become delaminated by multiple particle impacts during the erosion tests.

Erosion damage in diamond coatings by high velocity sand impacts

For a diamond-coated component, the shear stresses at the coating–substrate interface, generated by solid particle impingement, are known to affect interfacial integrity. If these stresses are of sufficient magnitude, coating-debonding caused by interfacial crack propagation can be initiated, which can later lead to catastrophic failure of the coating. This paper describes a set of experiments conducted on CVD diamond coatings at a constant particle impingement velocity (250 m/s), using sieved silica sand varying in diameter from 125 to 500 µm. The objective of this work was to examine the influence of the stress field on the integrity of the coating by varying the depth at which the maximum shear stress occurred. Detailed studies of the coating failure time with respect to the normalized depth of maximum shear stress show that particle impacts generating a maximum shear stress at, or close to, the coating–substrate interface results in rapid debonding of the coating. Coatings thick enough to contain the maximum shear stress within the coating and away from the interface exhibit the longest life when subjected to solid particle impacts. The results are also compared to other erosion studies and the differences between them are explained.

The erosion of CVD diamond by diamond particles

This paper describes the damage features generated by the high-velocity impact of diamond particles on diamond produced by chemical vapour deposition (CVD). Two types of diamond grit - angular and cubo-octahedral - were used to impact the surface of the diamond target at a mean impact velocity of 268 m s−1. The main damage features were elastic-plastic radial and lateral cracks. Distorted Hertzian ring cracks were also seen on samples impacted with cubo-octahedral diamond grit. This indicates that, in contrast to previous experiments by the authors using silica sand erodent, Hertzian damage can be generated at this velocity when the mechanical properties of the erodent and target are comparable. Fragmentation of the diamond grit occurred on impact; however, the extent of the degradation was much reduced compared with the silica sand erodent.