High-velocity air-fuel (HVAF) is a combustion process that allows solid-state deposition of metallic particles with minimum oxidation and decomposition. Although HVAF and cold spray are similar in terms of solid-state particle deposition, slightly higher temperature of HVAF may allow further particle softening and in turn more particle deformation upon impact. The present study aims to produce dense Ti-6Al-4V coatings by utilizing an inner-diameter (ID) HVAF gun. The ID gun is considered a scaled-down version of the standard HVAF with a narrower jet, beneficial for near-net-shape manufacturing. To explore the potential of the ID gun in the solid-state deposition process, an investigation was made into the effect of spraying parameters (i.e., spraying distance, fuel pressure, and nozzle length) on the characteristics of in-flight particles and the attributes of the as-fabricated coating such as porosity, oxygen content, and hardness. Using online diagnostics to monitor temperature and velocity of in-flight Ti-6Al-4V particles is challenging due to exothermic oxidation reaction of fine particles, while larger particles are too cold to be detected from their thermal emission. However, DPV diagnostic system was successfully employed to differentiate the non-emitting solid particles from the burning ones. It was found that increasing air and fuel pressure of the ID-HVAF jet led to an increase of the velocity of the in-flight particles, and resulted in improved density and hardness of the as-sprayed samples. However, increasing the spraying distance had a negative effect on the density and hardness of the deposits. It was also observed that the phases of the Ti-6Al-4V deposits were altered by producing vanadium oxide due to the high temperature of the spray jet.

The aim of this work is to study the effects of the titanium plasma spray (TPS) coating process on the fatigue resistance of a titanium-6Al-4V substrate. The combination of TPS processes and Ti alloy substrate is widely applied on components intended for cementless total hip replacement (THR). In order to understand the coating process mechanism behind the implants’ fatigue resistance decrease, one air-developed coating (Ti-APS) and one controlled atmosphere developed coating (Ti-CAPS) were considered. The effects of the most representative parameters of the plasma spray process on the fatigue resistance were analysed: the sandblasting process, the plasma and the coating powder. Fatigue resistance studies were performed by means of rotating bending fatigue testing. After fatigue failure specimens underwent morphological analyses both on the primary crack surface and on the cross-sectional area complemented by of the metallographic analyses of the coating. The titanium substrate fatigue resistance decreased after being blasted with direct relationship with the grain size. Ti-CAPS process showed a relatively limited further influence on the fatigue resistance reduction with respect to only sandblasted samples. By contrary a remarkable fatigue limit decreased was seen for Ti-APS coated samples against Ti-CAPS and simply sandblasted samples. The experiment pointed out the critical importance of cracks oxidation as a fatigue failure driving factor.

Anisotropy of stress-strain behavior, fracture toughness, and fatigue crack growth rate of Ti6Al4V deposited by cold spray using nitrogen was studied. For that, flat deposits were tested with stress acting in the in-plane directions and tubular deposits were tested in the out-of-plane stress directions. In all tests, unified small-size specimens were used. It was shown that for the in-plane stress, the deposits can be considered isotropic, whereas the out-of-plane stress led to significantly lower values of the measured properties. The obtained results were related to fractography and microstructural analysis. While a combination of trans-particle and inter-particle fracture determined the fatigue properties in the near-threshold regime, at higher loads, inter-particle fracture was dominant. It was also shown that the different particle-to-stress orientations influenced the resulting fatigue and static properties.

Metallic implants for orthopedic or dental use are often coated with a plasma-sprayed hydroxyapatite (HA) layer. In this study, HA coatings are applied to titanium substrates of varying thickness and laser shock adhesion tests are performed using different laser spot diameters. The objective is to investigate the effect of different shockwave regimes on interfacial debonding and the potential consequences of laser shock adhesion testing. HA coatings exhibiting different levels of adhesion were subjected to laser shock experiments and subsequently examined using nondestructive inspection techniques. The results are presented along with suggestions for developing a robust laser shock adhesion test.

In this study, Ti-6Al-4V coatings were deposited by low-pressure plasma spraying (LPPS) using different powder sizes and spray currents. The coatings were also heat treated at different temperatures, after which their microstructure and properties were assessed. The results show that finer powders are more conducive to the preparation of dense coatings and that porosity is reduced by increasing plasma current. As for the effects of heat treating temperature, at 870 °C, the lamellar structure of the coating disappeared and was found to be fully equiaxed with a grain size of 5-10 µm at 1100 °C. Hardness also increased, becoming significantly higher than that of forged TC4 alloy following treatment at 1100 °C.

In this work, hot isostatic pressing (HIP) is used to reduce interior defects, adjust the microstructure, and improve the tensile properties of cold-sprayed Ti6Al4V. Optical microscope and X-ray tomography were used to characterize pore morphologies and porosity evolution. XCT reconstructions show that fully dense Ti6Al4V alloy with an equiaxed microstructure were achieved. Tensile testing shows that strength and ductility were improved as well because of enhanced diffusion and resultant metallurgical bonding.

Phase composition and microstructure of hydroxyapatite (HA) significantly affects the biological and mechanical properties of final hydroxyapatite (HA) coating. In the present study, HA coatings were deposited on Ti-6Al-4V by micro-plasma spraying (MPS) using different spray parameters. The influence of spray parameters on the composition and microstructure of the coatings were investigated. To understand the formation mechanism of HA coatings, the in-flight particles and splats were examined as well. The morphologies of coatings surface, cross-sections, initial powder, in-flight particles and splats were characterized by scanning electron microscopy (SEM). Xray diffraction (XRD) was employed to analyze the phase composition. Three typical HA coatings were fabricated. The results indicated that the coating composition and microstructure were tightly related to the melting state of inflight particles. And this was influenced by the spraying parameters. The formation mechanisms of these coatings were discussed.

Wollastonite coatings were deposited on Ti6Al4V substrate at the substrate temperatures of room temperature, 200 °C, 400 °C and 600 °C by the atmosphere plasma spraying, respectively. The effect of substrate temperature on the microstructure, phase composition, mechanical properties and dissolution behavior of wollastonite coatings were investigated. The microstructure and phase composition of coatings were examined by SEM and XRD. The hardness and elastic modulus were obtained by the Knoop indentation tests. In addition, the dissolution behavior of coatings was evaluated by immersion in SBF solution. The results indicate that a slight decrement of porosity and an obvious increment of crystallinity were found with the substrate temperature. The hardness and elastic modulus of coatings increased with the substrate temperature up to 400 °C firstly, and then a decrement was observed with the temperature further increasing to 600 °C. The dissolution rate of coatings characterized by the pH changes and the released Ca, Si and P concentration in the SBF decrease with the substrate temperature, which is related to the porosity and crystallinity of coatings. It is revealed that through increasing the substrate temperature during plasma spraying is a feasible method to improve the mechanical properties and to decrease the dissolution of wollastonite coatings.

This study reports on the effect of combined pulsed laser ablation and laser pre-heating surface pre-treatments to cold spraying Ti and Ti-6Al-4V on coatings’ microstructure, bond strength and cohesive strength. The Ti and Ti-6Al- 4V coatings were sprayed on pure titanium and Ti-6Al-4V substrates, respectively. Coatings were characterized by SEM and porosity level was evaluated through image analysis. Bond strength was evaluated by standard ASTM C633 pull tests and by the laser shock (LASAT) technique. Cohesive strength was evaluated by the cross-section scratch test method. Results show that among the spray conditions used in this study, laser pre-treatment yielded high bond strength (such that all cases had higher cohesive strength than the epoxy glue). The LASAT technique provided a means to evaluate the influence of the laser ablation energy density and the laser pre-heating temperature. For both Ti and Ti-6Al-4V coatings, surface pre-heating increased the coating bond strength to the substrate. The laser ablation process would either increase or decrease the bond strength of the coating to the substrate depending on the laser energy density. The laser energy density needs to be adjusted as a function of the surface pre-heating temperature in order to optimize bond strength improvement. Coating cohesion did not improve with continuous laser pre-treatment in-between passes. However, the laser pre-heating helped reduce the coating porosity.

The fretting phenomenon was investigated experimentally in contacts between nitrided, coated and nitrided-coated Ti-6-4 rods against uncoated M50 rods generating a circular Hertzian contact. A fretting wear test rig was designed and developed to facilitate mounting of rod specimens of different coating thicknesses. Fretting wear tests were performed on low temperature and high temperature nitrided Ti-6-4 rods as well as on T-800 (CoCrMoSi) thermal spray coated Ti-6-4 and T-800 coated M50 rods. Finally, tests were carried out on Ti-6-4 rods nitrided at low and high temperatures and T-800 thermal spray coated on the top. The results obtained from fretting tests of each surface against uncoated M50 are studied and compared. Fretting wear volumes and surface profiles are presented for the contacts studied. The fretting wear resistance of each surface is quantified and compared with Archard’s wear equation. The role of amplitude of motion and number of cycles on the fretting wear of coatings is discussed. It was observed that increase in fretting wear resistance of uncoated Ti-6-4 rods by nitriding is greater than thermal spray coating. The fretting wear resistance was found to be higher for high temperature nitrided-coated rods than for low temperature nitrided-coated rods.

Hydroxyapatite (HA) is preferred for its ability to interact with living bone, resulting in improvements of implant fixation and faster bone healing. In this study, a small amount of silicon dioxide (~ 2wt%) was introduced into HA slurry which was subsequently spray-dried into powder. A silicon modified HA coating was then deposited onto Ti-6Al-4V alloy substrates by atmospheric plasma spraying technology. Scanning electron microscopy (SEM), X-ray diffraction and X-ray photoelectron spectrometry, and Raman spectrometry were employed to investigate the surface chemistry that would directly influence bone forming cell proliferation. Additionally, the adhesive bonding strength of the as-sprayed coatings were specified measured using a universal testing system. The fracture surfaces after tensile test were also investigated by SEM.

Chemical reactions between melted materials and their gaseous environment are generally inherent to the thermal spray process. Measures to promote and control these reactions are the distinguishing characteristic of reactive plasma spraying. In this paper, Ti-6Al-4V nitrided coatings are produced by high-pressure reactive plasma spraying. The coatings are deposited at different pressures up to 250 kPa in a reactive nitrogen atmosphere as well as air in order to study the influence of spraying pressure and atmosphere. The microstructure and phase composition of the Ti-6Al-4V layers are examined with the aid of X-ray analysis, microprobe measurements, and electron imaging. The investigations show that the pressure-supported nitrogen application during spraying led to the formation of fine and coarse TiN in the Ti matrix. Paper includes a German-language abstract.

This paper compares two types of hydroxyapatite (HA) composite coatings, HA/Ti-6Al-4V and HA/Y-ZrO2. The powders used in the study were prepared using a slurry process then deposited by plasma spraying. The resulting coatings were characterized based on their microstructure, mechanical properties, and biocompatibility. Both composite coatings performed better than pure HA coatings in tensile adhesion and indentation tests. Testing also revealed that the HA/Y-ZrO2 coatings had favorable strength and fracture toughness and that the HA/Ti-6Al-4V coatings had good affinity to living tissue and sufficient mechanical strength.

The growing need for new materials and material combinations with superior properties for severe service applications has led to the development of near net-shape forming techniques for certain materials, such as superalloys, refractory metals (Ta, W, and Mo) and highly reactive metals (Ti and its alloys). Vacuum plasma spray (VPS) was used to produce dense Ti-6Al-4V deposits for mechanical properties evaluation. Spherical Ti-6Al-4V powder, produced by Plasma Atomization (PA), a novel patented powder fabrication technique, was used as the starting powder. Plasma atomized Ti-6Al-4V powder characteristics include: high purity, tight particle size range, highly spherical with no attached satellites, and excellent flowability. The resulting as-sprayed Ti-6Al-4V deposits were dense and low in oxygen content. Thermal treatment was conducted after spraying in order to improve the structure and the properties of the spray formed material. The mechanical properties of the material, including tensile strength, elongation and hardness, in both the as-sprayed and the heat treated conditions were compared. The mechanical properties of these preliminary VPS Ti-6Al-4V specimens indicate that the combination of high purity starting powder and controlled environment deposition can be used to produce dense spray formed Ti-6Al-4V structures with properties comparable to those of cast or sintered powder metallurgy parts.

Bond coats based on bioinert ceramic materials such as titania and zirconia were developed to increase the adhesion strength of the coating system hydroxyapatite/bond coat to Ti6A14V alloy surfaces used for hip endoprostheses and dental root implants. The bond coats improved the adhesion strength, measured by a modified ASTM D3167-76 peel test, by 50 to 100% and also the resorption resistance as determined by in vitro leaching in simulated body fluid (Hank's Balanced Salt Solution, HBSS) for up to 28 days.

A new plasma atomization process was developed for the production of spherical metal powders. This process, unique in its ability to produce large quantities of fine powders (< 75 µm), is especially interesting for value added products such as refractory or reactive metals/alloys powders. The excellent feeding characteristics of spherical powders make them ideal for applications such as injection molding and vacuum plasma spray (VPS) deposition. Powders are known to have a significant influence on the final characteristics of VPS deposited coatings. A variety of factors will control the coating quality, including: powder chemistry, morphology, microstructure, and feeding behaviour. This article includes a review of titanium coatings, and new results obtained using plasma atomized spherical powders.

One cause of aseptic loosening of total hip replacement (THR) results from the hard polymetylmethacrylate (PMMA) bone cement debris embedded within some of acetabular cups [1]. To prevent this failure, PMMA can be replaced by a titanium coating with a specific roughness to promote the bone bonding. In this work, inert and atmospheric plasma spraying was used to coat Ti.6A1.4V implants with titanium.] In order to evaluate the effect of the deposition process on the coating corrosion resistance, different electrochemical techniques were implemented in physiological (i. e., Ringer) and in acidic solutions. Results show that the spraying parameters and the coating morphology affect the corrosion behaviour. Thus, if the reactivity is not affected by the pH evolution, the pitting sensitivity depends on the process. It was found that coatings deposited under inert gas are free of oxides and dense, even when the atmosphere contains some hydrogen. In that case a significant improvement of the intrinsic localized corrosion resistance is observed versus deposits obtained using atmospheric plasma spraying.