Compressor abradables coming into operational contact with bare, un-tipped titanium alloy rotor blades over a wide range of incursion conditions require excellent cuttability in order to avoid blade tip damage by wear and over-heating. This is more easily achieved for low temperature systems that can make use of low shear strength aluminum matrices than for compressor abradables operating closer to the maximum allowable temperature of advanced titanium alloy blade materials. In this case the rotor path linings will have to incorporate higher temperature resistant Ni and Co alloy matrices. To that end the availability of abradable coatings capable of operating at up to 550°C while showing little thermal ageing effects and excellent abradability over their entire service life can influence the compressor blade material selection and therefore compressor weight and performance characteristics. This paper provides an overview of titanium blade friendly compressor abradable concepts. Particular emphasis will be placed on the abradability of in-service and next-generation coatings designed for use up to the temperature capability of Ti blade rotor materials and beyond. Candidate coatings are also screened for other performance criteria such as thermal cyclic resistance and ageing behaviour.

Thermal spraying covers a range of spraying processes in which any metallic, ceramic or polymer-based material can be sprayed onto a substrate to be protected. This flexibility of thermal spraying makes the technology extremely successful in different engineering applications. Protection of the metallic components by hard oxide or ceramic-metallic coatings has proven to be an effective way to reduce wear and corrosion. Polymer coatings are increasing in importance as protection against corrosion and wear for several environmental conditions. Polymeric materials typically exhibit a low shear strength and low elastic modulus that make them viable for use under tribological conditions. Thermally sprayed polymers have the potential both to lower material costs and improve coating performance in drastic environments. In this work, a comparative study is carried out to evaluate the performance of some thermoplastic and hydrocarbon derived polymers deposited onto steel by thermal spray technology. Several tests were performed to evaluate the obtained coatings mainly related to tribological performance.

The work deals with the evaluation of strength characteristics of thermally sprayed coatings. The main aim was concentrated on the tensile and shear loading of HVOF (Stellite Alloy 6, Tribaloy 400) and arc sprayed (13%Cr, CuAl8) coatings. The investigation of the coatings behaviour on the coating-substrate interface is important for the evaluation of one of significant coating mechanical properties that influence properties of the whole coating-substrate system. The magnitude of the coating bond strength during tensile and shear stresses predicates the coating stability, reliability, impact resistance, resistance against failure and mostly operating lifetime. The determination adhesive-cohesive strength was performed according to EN 582 and EN 15340 Standards.

The shear test in accordance with EN 15340 is a new test method for testing the bond between substrate and thermally sprayed coatings. It has been developed in order to enable a test method without the disadvantages of the method currently in use, the adhesion test. In the adhesion test the adhesives influence the test results; therefore in the shear test no adhesives are used. The adhesives are among others the reason for a large deviation of the test results using the pull-off test. Using the shear test the mode of the test results depend on the ratio between adhesion and cohesion; this ratio causes three different fracture modes. In order to investigate the deviation depending on the fracture mode samples have been coated by wire flame spraying, arc, plasma and HVOF spraying. Different ceramics, carbides and metals have been used as thermal spraying materials. For each material-process combination between 30 and 120 samples have been coated. This report describes the interpretation of the results of the shear test depending on the fracture mode and the coating materials applied by different thermal spraying processes. The deviation of the results depending on the fracture mode has been investigated using a shear test device by which the shear force is recorded over the displacement of the coating. The knowledge of the deviation and the distribution of the results is necessary to decide on the required number of samples to reach a result with a defined reliability.

Thermally sprayed Inconel 718 coatings have been deposited by high velocity oxy-fuel (HVOF) spraying on Inconel 718 substrates. The aim of the on-going study is to understand and control the adhesion mechanisms and the residual stress state of the deposit/substrate system, in order to build up thick coatings for maintenance purposes. The coating adhesion strength was evaluated by the standard ASTM C633 tensile test. Coating shear strength was evaluated by the recently developed prEN15340 Shear Test. A modified Layer Removal Method (MLRM) test was carried out to measure residual stresses. The work is a part of an ongoing study for evaluation of relationships between process parameters, residual stress distribution and adhesion strength.

Gas Dynamic Spray (GDS) is a high rate, direct material-deposition process that utilizes the kinetic energy of particles sprayed at supersonic velocities to cause bonding through the particle plastic deformation on impact. GDS seems to be similar to the powder shock consolidation process, which is governed by dynamic regimes of granular material deformation under impulse loading. These regimes are characterized by adiabatic shear band (ASB) formation. This paper describes the preliminary analysis of ASB formation during GDS on the basis of a combination of the Johnson-Cook and shock wave consolidation models. The dependence of the ASB width on different parameters, including initial powder porosity, average impact stress, shear strain, initial temperature, and contact time was determined. The ASB width was found to vary in the range of 0.5–15 μm, which reveals the great localization of particle deformation in the GDS process.

Three adhesion measurement methods for thermal spray coatings, namely tensile adhesive strength (according to EN 582), interfacial indentation and in-plane tensile tests were investigated in terms of accuracy of the results and application potential for different coating / substrate conditions. Whereas the tensile adhesive strength test is widely used in industry, the other two methods are still under development in research laboratories and therefore only few experimental data on the accuracy of the methods and on the potential in an industrial context are available. For that reason, dissimilar coating-substrate combinations covering a wide range of types of thermal spray coating-substrate systems were tested using all these methods. Ceramic (Al 2 O 3 ) and metallic (NiCr 80-20) coatings were thermally sprayed by flame spraying with two different thickness on titanium alloy and steel substrates exhibiting each two distinct roughness levels. The distinguished coating properties include the coating toughness, shear strength, interfacial toughness, and adhesive strength. Thermally sprayed coatings do not only show an interfacial complexity, but also the integrity of the interface of substrate and coating has to be considered, as well as porosity, cracks and residual stresses. In this paper, each measurement method was found to be related to certain type of loading conditions and fracture mode. The results of the different methods are compared and the limits of applicability of the different methods are discussed.

One of the largest successes of modern medicine is the total hip replacement. Presently this procedure has one of the highest success rate among surgical interventions, only second to the appendix removal procedure. However the lifetime of the prosthesis itself is still limited to 10 to 20 years, which means that for numerous patients replacement of the procedure will become mandatory. This replacement finds its origin in aseptic loosening of the prosthesis mainly caused by the formation of wear particles at articular joints and by the difference in stiffness between the bone and the metallic prosthesis leading phenomena called stress shielding. To overcome this problem, new designs of more biomimetic prostheses, with stiffness similar to that of cortical bone, are being studied. Among the latter, a novel design based on polymer composite materials of total hip replacement prosthesis is under development. One of the key characteristics of this biomimetic prosthesis is its hydroxyapatite coating, which permits Osseo integration (integration into the bone). Thermally sprayed hydroxyapatite coatings are already used successfully for metallic implants, but plasma sprayed hydroxyapatite coatings have yet to be developed for polymer composites due to quite challenging heat management and adhesion concerns. This paper describes and discusses the optimization of the plasma sprayed technique and the formation of the adequate underlayer enabling the plasma spray on highly heat sensitive substrate. Adhesion, shear and fatigue results are presented. Abstract only; no full-text paper available.

Fracture strength of a WC-12Co thermal sprayed coating is investigated experimentally and analytically. In the experiments, a pair of coated butt cylindrical specimens is subjected to combined tension with torsion stresses. Stress distributions in the coating at critical loading conditions are analyzed by using FEM analysis and the fracture criterion of the coating is investigated. It is found that normal stress distributions are common to all cases of testing stress conditions and so fracture condition of the brittle coating is represented as K ≧ Kcr in the normal stress distribution even under mixed deformation mode I and III. By using the experimental method presented in this paper and the corresponding FEM analysis, an approximation for the critical bonding strength of a coating under shearing load can be estimated.

As ceramic materials, the use of hydroxyapatite (HA) in clinical applications is severely limited by its intrinsic poor mechanical properties. The incorporation of some bioinert ceramics is believed to be a way to improve the mechanical reliability of HA matrix. HA coatings with titania addition were produced by using high velocity oxy-fuel (HVOF) spraying process in the present study. The mechanical properties of the as-sprayed coatings in terms of adhesive strength, shear strength and fracture toughness were investigated aiming to reveal the reinforcing effect of the titania addition in HA coatings. Qualitative phase analysis through X-ray diffraction (XRD) showed that mutual chemical reaction between TiO 2 and HA occurred during coating formation, from which CaTiO 3 was resulted. Totally unmelted titania powders were observed which suggests that the mutual reaction locates at HA/TiO 2 splats' interface. Significant influence of coating microstructure on mechanical properties was revealed. As the content of titania in HA coatings reached 20vol%, the adhesive strength decreased largely. As the content of titania reached 30vol%, the further augmentation of the adhesive strength of is possibly resulted from the improved coating microstructure. The fracture toughness exhibited the values of 0.48 Mpa ⋅ m ½ 0.60 Mpa ⋅ m ½ and 0.67 Mpa ⋅ m ½ for pure HA coating, 10vol% TiO 2 blended HA coating and 20vol% TiO 2 blended HA coating, respectively.