The processing conditions, microstructural and tribological characterizations of plasma sprayed CoNiCrAlY-BN high temperature abradable coatings are reported in this manuscript. Plasma spray torch parameters were varied to produce a set of abradable coatings exhibiting a broad range of porosity levels (34-62%) and superficial Rockwell hardness values (0-78 HR15Y). Abradability tests have been performed using an abradable-seal test rig capable of simulating operational wear at different rotor speeds and seal incursion rates. These tests allowed determining the rubbing forces and quantifying the blade and seal wear characteristics for slow and fast seal incursion rates. Erosion wear performance and ASTM C633 coating adhesion strength test results are also reported. For optimal abradability performance, it is shown that coating hardness needs to be lower than 70 and 50 HR15Y for slow and fast blade incursion rate conditions, respectively. It is shown that the erosion wear performance, as well as, the coating cohesive strength is a function of the coating hardness. The current results allow defining the coating specifications in terms of hardness and porosity for targeted applications.

In this work the sliding wear resistance of HVOF and APS-sprayed coatings from two experimental TiO 2 -10-20 wt.%Cr 2 O 3 fused and crushed feedstock powders was investigated. During the spraying process the particle temperatures and velocities were mapped in the cross section of the spray jet at different spray distances. In addition, the coating surface temperature was monitored. Coating microstructures and phase composition were studied by SEM and XRD, respectively. The elastic modulus (E) and hardness (H) values were measured via in-depth sensing indentation. The unidirectional ball-on-disk sliding wear test was performed at room temperature against a tungsten carbide ball (sliding speed 0.05 m/s, normal force 90 N). The increase of the particle temperature and velocity leads to an increase of hardness and Young’s modulus of the coatings, thereby increasing their sliding wear resistance. The sliding wear resistance can be correlated with the H 3 /E 2 ratio better than with the hardness alone. The sliding wear resistance was also sensitive to the coating surface temperature reached during deposition.

Si-based ceramics (e.g., SiC and Si 3 N 4 ) are known as promising high-temperature structural materials in various components where metals/alloys reached their ultimate performances (e.g., advanced gas turbine engines and structural components of future hypersonic vehicles). To alleviate the thickness recess that Si-based ceramics undergo in a high-temperature environmental attack (e.g., H 2 O vapour), appropriate refractory oxides are engineered as environmental barrier coatings (EBCs). Presently, the state-of-the art EBCs comprise multilayers of silicon (Si) bond coat, mullite (Al 6 Si 2 O 13 ) intermediate layer and BaO-SrO-Al 2 O 3 -SiO 2 (BSAS) top coat. Evaluating and understanding their mechanical properties, such as, the elastic modulus (E) and the strain-stress relationship is essential for their practical application and reliable employment. It was investigated via depth-sensing indentation the role of high-temperature treatment (1300°C), performed in H 2 O vapour environment (for time intervals up to 500 h), on the mechanical behaviour of air plasma sprayed Si/mullite/BSAS layers deposited on SiC substrates. Laser-ultrasonics was employed to evaluate the E values of as-sprayed coatings and to validate the indentation results. The fully crystalline, crack-free and near crack-free as-sprayed EBCs were engineered under controlled deposition conditions. The (i) absence of phase transformation and (ii) stability of the low elastic modulus values (e.g., ~60-70 GPa) retained by the BSAS top layers even after harsh environmental exposures provides a plausible explanation for the almost crack-free coatings observed. The measured mechanical properties of the EBCs and their microstructural behaviour during the high-temperature exposure are discussed and correlated.

This study investigates the influence of powder morphology and spray processes on the microstructure, crystallinity, hardness, and elastic modulus of mullite coatings. Coatings produced from mullite powders and suspensions are deposited by plasma spraying while measuring in-flight particle temperature and velocity. Powder morphology and spraying conditions are correlated with measured coating properties, creating a process map for engineering mullite coatings for specific applications. It is shown that coating crystallinity, microstructure, and mechanical properties vary widely depending on powder morphology, processing, and in-flight particle characteristics.

In this study, two processing routes are used to produce mullite powders for thermal spraying and the influence of each method on particle morphology and microstructure is investigated. Different thermal treatments are performed to improve grain cohesion and powder flow and their effect on the crystal structure of the powder is assessed as well. The powders are plasma sprayed, in-flight characteristics are measured, and splats are collected and analyzed. A correlation among powder morphology, in-flight particle properties, and splat morphology is established to better understand the influence of powder processing route on coating formation.

In this study, titania and hydroxyapatite nanopowder mixtures are deposited on medical grade titanium substrates by HVOF spraying. To assess bioperformance, human mesenchymal stem cells (hMSCs) were cultured from 1 to 21 days on the surface of HVOF-sprayed TiO 2 and TiO 2 -HA samples. Plasma sprayed HA and uncoated Ti-6Al-4V substrates were used as controls. The active cultures were evaluated for cell proliferation, cytoskeleton organization, and cell-substrate interaction. The results for HVOF-sprayed TiO 2 -HA nanocomposite coatings show strong evidence of bone growth, proliferation, and attachment with cell-substrate interaction levels superior to those of air plasma sprayed HA coatings. Although there are no clear explanations for this favorable behavior, the topography and chemical composition of the surface of the coating appear to be playing important roles.

This study assesses the photocatalytic properties of HVOF-sprayed nanostructured TiO 2 coatings, particularly their bactericidal effect. The surfaces of the coatings were lightly polished before being exposed to bacterial solutions of known concentration. The solution was dispensed on the coating and irradiated with white light in 30-minute intervals up to 120 minutes. On polished HVOF-sprayed TiO 2 coatings, 24% of the bacteria were killed after 120 minutes of exposure. On stainless steel controls, the percentage of bacterial cells killed was approximately 6% for most of the exposure times studied.

Envisioning the use of nanostructured YSZ coatings at high temperatures cause concerns in the scientific community. Questions have been raised about the possibility of accelerated sintering of these ultra-fine materials and the associated changes in properties that could accompany this sintering. In this work, nanostructured YSZ coatings were engineered to counteract sintering effects by tailoring the coatings to exhibit a bimodal microstructure formed by (i) a matrix of dense YSZ zones (produced from molten YSZ particles) and (ii) large porous nanostructured YSZ zones (produced from semi-molten nanostructured YSZ particles) that were embedded in the coating microstructure during thermal spraying. These coatings were subjected to heat treatment in air at 1400°C for 1, 5 and 20 h. The superior driving force for sintering exhibited by the porous nanozones, when compared to that of the dense zones, caused the nanozones to shrink at much faster rates than those exhibited by the denser matrix zones (i.e., differential sintering), thereby creating a significant network of voids in the coating microstructure. Due to these effects, after 20 h exposure at 1400°C, the thermal conductivity and elastic modulus values of the conventional coatings were approximately two times higher than those of the nanostructured ones.

Titania (TiO 2 ) coatings are candidates for high-temperature applications in the fields of wear, corrosion, and environmental barrier coatings (EBCs); however, at temperatures at or above 540 °C, titania coatings are not pursued due to the usual presence of the anatase phase in the as-sprayed TiO 2 coatings. This phase tends to impede the applications of these materials at high temperatures due to the stresses provided by the critical anatase-to-rutile phase transformation at temperatures higher than 540 °C; such stresses tend to generate cracks in the coating microstructure, leading to premature coating failure. It has been hypothesized that this barrier could be overcome by the use of nanostructured TiO 2 coatings, due to their known high toughness and resilience levels. Nanostructured TiO 2 powders were HVOF-sprayed. The high velocity levels of the HVOF-sprayed particles generated a gas-tight microstructure (i.e., no through-thickness porosity). SEM pictures of the as-sprayed and heat-treated (800 °C for 1 h) coatings did not show any significant signs of crack network formation, which may have been prevented by the high toughness and resilience of these coatings. These coatings were also HVOF-sprayed on SiC substrates and did not exhibit macroscopic signs of delamination after a 1400 °C exposure for 1 h in air.

In previous studies, it has been demonstrated that nanostructured Al 2 O 3 -13wt%TiO 2 coatings deposited via air plasma spray (APS) exhibit higher wear resistance when compared to that of conventional coatings. This study aimed to verify if HVOF-sprayed Al 2 O 3 -13wt%TiO 2 coatings produced using hybrid (nano+submicron) powders could improve even further the already recognized good wear properties of the APS nanostructured coatings. According to the abrasion test results (ASTM G 64), there was an improvement in wear performance by a factor of 8 for the HVOF-sprayed hybrid coating as compared to the best performing APS conventional coating. When comparing both hybrid and conventional HVOF-sprayed coatings, there was an improvement in wear performance by a factor of 4 when using the hybrid material. The results show a significant anti-wear improvement provided by the hybrid material. Scanning electron microscopy (SEM) at low/high magnifications showed the distinctive microstructure of the HVOF-sprayed hybrid coating, which helps to explain its excellent wear performance.

Thermal barrier coatings were produced using both Ar and N 2 as the primary plasma gas. Various aspects of the process and the coatings were investigated. It was found that higher in-flight particle temperatures could be produced using N 2 , but particle velocities were lower. Deposition efficiencies could be increased by a factor of two by using N 2 as compared to Ar. Coatings having similar values of porosity, hardness, Young’s modulus and thermal diffusivity could be produced using the two primary gases. The coatings exhibited similar changes (increased hardness, stiffness and thermal diffusivity) when heat-treated at 1400°C. The results point to the potential advantage, in terms of reduced powder consumption and increased production rate, of using N 2 as compared to Ar as the primary plasma gas for TBC deposition.

Nanostructured titania (TiO 2 ) coatings were produced by high velocity oxy-fuel (HVOF) spraying. They were engineered as a possible candidate to replace hydroxyapatite (HA) coatings produced by air plasma spray (APS) on implants. They exhibited mechanical properties, such as hardness and bond strength, much superior to those of APS HA coatings. In addition to these characteristics, the surface of the nanostructured coatings exhibited regions with nanotextured features originating from the semi-molten nanostructured feedstock particles. This nanotexture is considered an asset, due to its better interaction with the adhesion proteins of the osteoblast cells, such as fibronectin, which exhibit dimensions in the order of nanometers. Osteoblast cell culture demonstrated that this type of coating supported osteoblast cell growth and did not negatively affect cell viability.

The aging baby boomer population coupled with an increase in life expectancy is leading to a rising number of active elderly persons in occidental countries. As a result, the orthopedic implant industry is facing numerous challenges such as the need to extend implant life, reduce the incidence of revision surgery and improve implant performance. This paper reports results of an investigation on the bioperformance of newly developed coating-substrate systems. Hydroxyapatite (HA) and nano-titania (nano-TiO 2 ) coatings were produced on Ti-6Al-4V and fiber reinforced polymer composite substrates. In vitro studies were conducted in order to determine the capacity of bioactive coatings developed to sustain osteoblast cells (fetal rat calvaria) adherence, growth and differentiation. As revealed by SEM observations and alkaline phosphatase activity (ALP), cell adhesion and proliferation demonstrated that HA coatings over a polymer composite are at least as good as HA coatings made over Ti-6Al-4V substrate in terms of osteoblast cell activity. Nano-TiO 2 coatings produced by high-velocity oxy fuel (HVOF) spraying led to different results. For short term cell culture (4.5 and 24 hrs), the osteoblasts appeared more flattened when grown on nano-TiO 2 than on HA. The surface cell coverage after 7 days of incubation was also more complete on nano-TiO 2 than HA. Preliminary results indicate that osteoblast activity after 15 days of incubation on nano-TiO 2 is equivalent to or greater than that observed on HA.

Retaining non-melted nano-particles of zirconia in nanostructured coatings has been a challenge in the past. Recently an air plasma spray process was developed to produce coatings which retain up to 30-35% by volume non-melted particles, resulting in a unique structure. The creep/sintering behavior of such thermal barrier coatings deposited from nanostructured feedstock has been measured and compared with deposits produced from hollow sphere powder (HOSPTM). Both feedstocks contain 6-8wt% Y 2 O 3 as stabilizer. Flexure and compression creep testing were conducted under several different loads and temperatures to obtain creep exponents and parameters.

New and more demanding applications and higher performance requirements are creating the need for a greater degree of sophistication in engineering coating structures. The use of nanostructured feedstocks provides the possibility of tailoring the structure of thermal spray ceramic-based coatings at the nanoscale. In the present study, it has been found that such an approach can produce coatings with enhanced mechanical, thermal and bioperformance characteristics. It has been shown that the internal structure and external size of agglomerates as well as the spray conditions employed for deposition play a key role in determining the nature and extent of zones of nanostructured material produced in coatings. The characteristics (such as porosity and bonding) of these zones can have an important effect on the coating performance. This approach has been used to tailor Al 2 O 3 -TiO 2 , ZrO 2 -Y 2 O 3 , WC-Co, TiO 2 , and hydroxyapatite coatings targeted for use as abradables, for TBCs, against wear and on orthopedic implants.

Nanostructured yttria stabilized zirconia abradable coatings were produced via air plasma spray (APS) and compared to conventional abradable coatings based on the composite CoNiCrAlY+BN+polyester also sprayed via APS. The microstructures of the coatings were analyzed using SEM and the hardness determined via Rockwell Y measurements. It was possible to engineer nanostructured abradables with a high degree of plasticity by controlling the amount, morphology and distribution of the nanostructured phase embedded in the coating microstructure. Room temperature rub-rig tests were performed for both types of coatings under different blade tip speeds and seal incursion rates simulating operating conditions of gas turbines. The nanostructured abradables exhibited good performance indicating that abradable coatings engineered in this fashion have potential for industrial application at elevated temperatures.

Titania (TiO 2 ) coatings were produced using the high velocity oxy-fuel (HVOF) technique on Ti-6Al-4V substrates. The titania feedstock powder exhibited nanostructured morphology, formed by the agglomeration of individual nanostructured titania particles (spray-drying) smaller than 100 nm. The resulting coatings were dense (porosity <1%) and exhibited rutile and anatase as phases with percentages of ~75% and ~25%, respectively. These coatings were heat-treated in a H 2 /N 2 environment at 700oC for 8 h. During the heat-treatment, nanostructured titania fibers were formed on specific surface regions of the coatings. The nanofibers formed by this “chemical or reaction-based texturing” exhibited diameters of 50-400 nm and lengths in the order to 1-5 µm. It is thought that engineering these surfaces at nano and microscales may lead to interesting applications of titania coatings related to cell attachment/growth (for biomedical applications).

In order to characterize the performance of nanostructured coatings during wear, nanostructured and conventional titania (TiO 2 ) coatings were sprayed via three different thermal spray processes: APS, VPS and HVOF. Three distinct types of wear resistance were evaluated: dry-abrasion and slurry-erosion at 30° and 90°. The ranking of the wear performance of the different coatings varied for the three wear tests, except for the HVOF-sprayed nanostructured titania. The HVOF-sprayed nanostructured coating exhibited the highest wear resistance in all three types of wear. The different angles of erosion (30° and 90°) did not cause a change in the wear performance (ranking) of the HVOF-sprayed nanostructured coating, as was observed for the other coatings tested. The superior mechanical performance of the HVOF-sprayed nanostructured titania can be explained by observing the microstructure of the coating via high magnification SEM. These observations show that the nanostructured zones in the coating microstructure act as crack arresters, thereby increasing the coating toughness. The wear scars for the different coatings were also analyzed via SEM and used to help understand the wear performance of the different materials.

Nanostructured and conventional titania (TiO 2 ) powders were thermally sprayed using APS and HVOF processes. The fatigue and mechanical properties of the coatings investigated. Coatings were characterized using SEM to investigate the microstructural features and Vickers indentation to determine the hardness. SEM analyses were also carried out on the fracture surfaces of fatigue-tested samples to assess crack nucleation and to study the mechanisms of deformations. The fatigue strength of coatings deposited onto low-carbon steel (AISI 1018) showed that the nanostructured titania coated specimens exhibited significantly higher fatigue strength compared to the conventional titania

Alumina-titania coatings deposited by air plasma spraying (APS) are widely used to protect components against wear at low temperatures. It is known that microstructures formed by the post-laser remelting of as-sprayed coatings exhibit a densification but also numerous macrocracks due to the rapid cooling and thermal stresses. By using the laser-assisted air plasma spraying (LAAPS), the laser beam interacts simultaneously with the plasma torch in order to increase coating surface temperature and possibly superficially remelt the coating. As a result, the microstructure is partially densified and macrocracks, which are generally produced in the post-laser irradiation treatment, can be inhibited. In addition, this hybrid spraying can be done without the post-treatment of coating. In this paper, LAAPS was performed to improve the mechanical properties of Al 2 O 3 -13%TiO 2 coatings. The coating microstructure was characterized by SEM and X-ray diffraction. The mechanical characterization was done by hardness measurements, erosive wear tests and abrasion wear tests. Results showed that laser assistance may induce: (1) the disappearance of vertical and horizontal macrocracks due the laser irradiation in the coatings for a laser irradiation density lower than 34 W.mm-2, (2) an important decrease of the amount of microcracks in the deposited splats, (3) the partial transformation of the metastable γ-Al 2 O 3 phase in the equilibrium α-Al 2 O 3 phase, (4) a hardness increase of 11%, (5) an improvement of the erosive wear resistance by 12% and (6) an improvement of the abrasive wear resistance by 38%.