HVOF-Sprayed Coatings Engineered from Mixtures of Nanostructured and Submicron Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> Powders: An Enhanced Wear Performance

In previous studies, it has been demonstrated that nanostructured Al₂O₃-13 wt.%TiO₂ coatings deposited via air plasma spray (APS) exhibit higher wear resistance when compared to that of conventional coatings. This study aimed to verify if high-velocity oxy-fuel (HVOF)-sprayed Al₂O₃-13 wt.%TiO₂ 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 antiwear 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. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Development of a Thermal Transport Database for Air Plasma Sprayed ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Thermal Barrier Coatings

Thermal diffusivities of air plasma sprayed (APS) thermal barrier coatings (TBCs) were measured by the laser flash method. The data were used to calculate thermal conductivity of TBCs when provided with density and specific heat data. Due to the complicated microstructure and other processing-related parameters, thermal diffusivity of TBCs can vary as much as three- to four-fold. Data collected from over 200 free-standing ZrO₂-7-8wt.%Y₂O₃ TBCs are presented. The large database gives a clear picture of the expected “band” of thermal diffusivity values. When this band is used as a reference for thermal diffusivity of a specific TBC, the thermal transport property of the TBC can be more precisely described. This database is intended to serve researchers and manufacturers of TBCs as a valuable resource for the evaluation of TBCs.     

Mechanical Properties of LaTi<Subscript>2</Subscript>Al<Subscript>9</Subscript>O<Subscript>19</Subscript> and Thermal Cycling Behaviors of Plasma-Sprayed LaTi<Subscript>2</Subscript>Al<Subscript>9</Subscript>O<Subscript>19</Subscript>/YSZ Thermal Barrier Coatings

Recently, extensive efforts have been made to develop new thermal barrier coating (TBC) materials which can operate at temperatures above 1523 K over a long term. In this article, LaTi₂Al₉O₁₉ (LTA) was synthesized by solid-state reaction at 1773 K, and the mechanical properties of the LTA bulk were evaluated. The microhardness is about 14 GPa, comparable to that of YSZ bulk, whereas the Young’s modulus is about 44 GPa, lower than the value of YSZ. However, the fracture toughness of 0.8-1 MPa m^1/2 is much lower than that of bulk YSZ. A double-ceramic-layer LTA/YSZ TBC structure was proposed and the TBC sprayed by plasma spraying. Thermal cycling tests of the TBC specimens were performed at 1373 K with a dwell time of 10 min. The LTA remained good stability with ZrO₂ and Al₂O₃. However, the single layer LTA TBC was cracked at the LTA/bond coat interface after about 300 cycles, due to its poor thermal shock resistance, while the YSZ TBC yielded a lifetime of about 1000 cycles. The LTA/YSZ TBC remained intact even after 3000 cycles, exhibiting a promising potential as new TBC materials.     

Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-NiCr HVOF-Sprayed Coatings: Microstructure and Properties Versus Powder Characteristics and Process Parameters

Two 75%Cr₃C₂-25%NiCr feedstock powders with the same size distribution but different production process were characterized and found quite different in terms of morphology and phase composition. The powders were sprayed in a HVOF Diamond Jet (Sulzer Metco DJ-2600) torch with five different values of the oxygen-to-hydrogen ratio in order to assess the influence of this parameter on the microstructure and properties of the coatings. The results show that the closed and dense microstructure of one powder (Woka 7302) results in coatings with lower amount of decarburization, less oxide formation and higher toughness compared to coatings from the other powder (Praxair 1375). It was found that the O₂/H₂ ratio impacts mainly on the Young’s modulus, which almost doubled by changing the ratio from 0.40 to 0.50, and on toughness, but does not notably affect the Vickers hardness.     

Phase formation of nano-TiO<Subscript>2</Subscript> particles during flame spraying with liquid feedstock

The nanostructured TiO₂ photocatalytic coatings were synthesized through flame spraying with liquid feedstock under different conditions. The nanostructured TiO₂ deposit of substantial anatase phase was annealed at different temperatures. X-ray diffraction analysis showed that significant transformation from anatase to rutile occurred at a temperature above 600 °C. However, thermal analysis suggested that the phase transformation from anatase to rutile started at a temperature from 400 to 500°C. It was found that the grain size of rutile phase was larger than that of anatase. The deposits annealed at temperatures lower than 450°C were photocatalytically active. However, the deposit annealed at 500°C, which contained 95% anatase crystalline, became photocatalytically inactive. Based on the experimental findings, a model is proposed to explain the phase transformation of the nano-TiO₂ particles and the phase formation in flame-spraying of nanostructured TiO₂ deposit with liquid feedstock. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

Thermal Spray Deposition,Phase Stability and Mechanical Properties of La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript>/LaAlO<Subscript>3</Subscript> Coatings

This paper deals with the deposition of La₂Zr₂O₇ (LZO) and LaAlO₃ (LAO) mixtures by air plasma spray (APS). The raw material for thermal spray, single phase LZO and LAO in a 70:30 mol.% ratio mixture was prepared from commercial metallic oxides by high-energy ball milling (HEBM) and high-temperature solid-state reaction. The HEBM synthesis route, followed by a spray-drying process, successfully produced spherical agglomerates with adequate size distribution and powder-flow properties for feeding an APS system. The as-sprayed coating consisted mainly of a crystalline LZO matrix and partially crystalline LAO, which resulted from the high cooling rate experienced by the molten particles as they impact the substrate. The coatings were annealed at 1100 °C to promote recrystallization of the LAO phase. The reduced elastic modulus and hardness, measured by nanoindentation, increased from 124.1 to 174.7 GPa and from 11.3 to 14.4 GPa, respectively, after the annealing treatment. These values are higher than those reported for YSZ coatings; however, the fracture toughness (K _IC) of the annealed coating was only 1.04 MPa m^0.5.     

HVOF thermal spray deposited Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-stabilized ZrO<Subscript>2</Subscript> coatings for thermal barrier applications

High velocity oxy-fuel (HVOF) thermal spray has been successfully used to deposit yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications. Adherent coatings were obtained within a limited range of spray conditions using hydrogen as fuel gas. Spray parameters such as hydrogen-to-oxygen ratio, spray distance, and substrate cooling were investigated. Spray distance was found to have a pronounced effect on coating quality; adherent coatings were obtained for spray distances between 75 and 125 mm from the gun exit for the hydrogen-to-oxygen ratios explored. Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density, surface roughness, and cross-sectional microhardness. Notably, fracture surfaces of the HVOF coatings revealed a more homogeneous structure. Many theoretical models predict that it should not be possible to melt YSZ in an HVOF flame, and therefore it should not be possible to deposit viable YSZ coatings by this process. The experimental results in the present work clearly contradict those expectations. The present results can be explained by taking into account the effect of partial melting and sintering on particle cohesion, as follows. Combustion chamber pressures (P _o) of ∼3.9 bar (58.8 psi) realized during HVOF gun operation allows adiabatic flame temperature values that are above the zirconia melting temperature. Under these conditions, the Ranz-Marshall heat transfer model predicts HVOF sprayed particle surface temperatures T _p that are high enough for partial melting of small (∼10 μm) zirconia particles, T _p=(1.10−0.95)T _m. Further analysis shows that for larger particles (38 μm), adherent coatings are produced when the particle temperature, T _p=0.59−0.60 T _m, suggesting that sintering may have a role in zirconia particle deposition during HVOF spray. These results suggest two different bonding mechanisms for powders having a broad particle size distribution.     

Properties and Performance of High-Purity Thermal Barrier Coatings

It has been found that reducing the level of impurity oxides (particularly SiO₂ and Al₂O₃) in 7YSZ, from about 0.2 wt% to below 0.1 wt% raises the sintering resistance and the phase stability of plasma-sprayed coatings. The implications for the usage of these coatings at elevated temperatures are examined. It is concluded that using relatively high-purity powder of this type is likely to confer substantial benefits in terms of the thermomechanical stability of the coatings under service conditions. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Influence of Annealing on Photocatalytic Performance and Adhesion of Vacuum Cold-Sprayed Nanostructured TiO2 Coating

Composite powder was prepared using primary nanoTiO₂ powder and polyethylene glycol (PEG). The nanoTiO₂ coating was deposited through vacuum cold spray using both the composite powder and the primary nanopowder. The influence of annealing on the coating adhesion and photocatalytic activity was investigated. The coating adhesion was evaluated through erosion test by water jet. The photocatalytic performance of the coatings was evaluated through photodegradation of phenol in water. Results showed that annealing of the coating at a temperature from 450 to 500 °C yielded both higher activity and better adhesion. The adhesion of the coating deposited using the composite powder was better than that using the primary nanoTiO₂ powder. It was found that the TiO₂ coating, resulting from the composite powder, presented much higher activity than that deposited with the primary nanopowder. The better activity is attributed to the existence of large pores resulting from the stacking of composite powder, which benefits the reactants’ transportation through the porous coating. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Photoresponse and donor concentration of plasma-sprayed TiO<Subscript>2</Subscript> and TiO<Subscript>2</Subscript>-ZnO electrodes

The photoelectrochemical characteristics of plasma-sprayed porous TiO₂, TiO₂-5%ZnO, and TiO₂-10%ZnO electrodes in 0.1 N NaOH solution were studied through a three-electrode cell system. The microstructure, morphology, and composition of the electrodes were analyzed using an electron probe surface roughness analyzer (ERA-8800FE), scanning electron microscopy, and x-ray diffraction. The results indicate that the sprayed electrodes have a porous microstructure, which is affected by the plasma spray parameters and composition of the powders. The TiO₂-ZnO electrodes consist of anatase TiO₂, rutile TiO₂, and Zn₂Ti₃O₈ phase. The photoresponse characteristics of the plasma-sprayed electrodes are comparable to those of single-crystal TiO₂, but the breakdown voltage is close to 0.5 V (versus that of a saturated calomel electrode). The short-circuit photocurrent density (J _SC) increases with a decrease of donor concentration, which was calculated according to the Gartner-Butler model. For the lowest donor concentration of a TiO₂-5%ZnO electrode sprayed under an arc current of 600 A, the short-circuit J _SC is approximately 0.4 mA/cm² higher than that of the TiO₂ electrodes under 30 mW/cm² xenon light irradiation. The J _SC increases linearly with light intensity. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

Microstructure and Thermal Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript> Coating

In the present work, Yb₂Si₂O₇ powder was synthesized by solid-state reaction using Yb₂O₃ and SiO₂ powders as starting materials. Atmospheric plasma spray technique was applied to fabricate Yb₂Si₂O₇ coating. The phase composition and microstructure of the coating were characterized. The density, open porosity and Vickers hardness of the coating were investigated. Its thermal stability was evaluated by thermogravimetry and differential thermal analysis (TG-DTA). The thermal diffusivity and thermal conductivity of the coating were measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb₂Si₂O₇ with amorphous phase. The coating had a dense structure containing defects, such as pores, interfaces and microcracks. The TG-DTA results showed that there was almost no mass change from room temperature to 1200 °C, while a sharp exothermic peak appeared at around 1038 °C in DTA curve, which indicated that the amorphous phase crystallized. The thermal conductivity of the coating decreased with rise in temperature up to 600 °C and then followed by an increase at higher temperatures. The minimum value of the thermal conductivity of the Yb₂Si₂O₇ coating was about 0.68 W/(m K).     

Effect of Nano-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> Additives and Plasma Treatment on the Dry Sliding Wear Behavior of Plasma Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-8YSZ Ceramic Coatings

In this paper, the effect of nano-Si₃N₄ additives and plasma treatment on the wear behavior of Al₂O₃-8YSZ ceramic coatings was studied. Nano-Al₂O₃, nano-8YSZ (8 wt.% Y₂O₃-stabilized ZrO₂) and nano-Si₃N₄ powders were used as raw materials to fabricate four types of sprayable feedstocks. Plasma treatment was used to improve the properties of the feedstocks. The surface morphologies of the ceramic coatings were observed. The mechanical properties of the ceramic coatings were measured. The dry sliding wear behavior of the Al₂O₃-8YSZ coatings with and without Si₃N₄ additives was studied. Nano-Si₃N₄ additives and plasma treatment can improve the morphologies of the coatings by prohibiting the initiation of micro-cracks and reducing the unmelted particles. The hardness and bonding strength of AZSP (Al₂O₃-18 wt.% 8YSZ-10 wt.% Si₃N₄-plasma treatment) coating increased by 79.2 and 44% compared to those of AZ (Al₂O₃-20 wt.% 8YSZ) coating. The porosity of AZSP coating decreased by 85.4% compared to that of AZ coating. The wear test results showed that the addition of nano-Si₃N₄ and plasma treatment could improve the wear resistance of Al₂O₃-8YSZ coatings.     

Superior performance of high-velocity oxyfuel-sprayed nanostructured TiO<Subscript>2</Subscript> in comparison to air plasma-sprayed conventional Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-13TiO<Subscript>2</Subscript>

Air plasma-sprayed conventional alumina-titania (Al₂O₃-13wt.%TiO₂) coatings have been used for many years in the thermal spray industry for antiwear applications, mainly in the paper, printing, and textile industries. This work proposes an alternative to the traditional air plasma spraying of conventional aluminatitania by high-velocity oxyfuel (HVOF) spraying of nanostructured titania (TiO₂). The microstructure, porosity, hardness (HV 300 g), crack propagation resistance, abrasion behavior (ASTM G65), and wear scar characteristics of these two types of coatings were analyzed and compared. The HVOF-sprayed nanostructured titania coating is nearly pore-free and exhibits higher wear resistance when compared with the air plasma-sprayed conventional alumina-titania coating. The nanozones in the nanostructured coating act as crack arresters, enhancing its toughness. By comparing the wear scar of both coatings (via SEM, stereoscope microscopy, and roughness measurements), it is observed that the wear scar of the HVOF-sprayed nanostructured titania is very smooth, indicating plastic deformation characteristics, whereas the wear scar of the air plasma-sprayed alumina-titania coating is very rough and fractured. This is considered to be an indication of a superior machinability of the nanostructured coating.     

Mechanical Properties of Layered La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> Thermal Barrier Coatings

Lanthanum zirconate (La₂Zr₂O₇) has been proposed as a promising thermal barrier coating (TBC) material due to its low thermal conductivity and high stability at high temperatures. In this work, both single and double-ceramic-layer (DCL) TBC systems of La₂Zr₂O₇ and 8 wt.% yttria-stabilized zirconia (8YSZ) were prepared using air plasma spray (APS) technique. The thermomechanical properties and microstructure were investigated. Thermal gradient mechanical fatigue (TGMF) tests were applied to investigate the thermal cycling performance. The results showed that DCL La₂Zr₂O₇ + 8YSZ TBC samples lasted fewer cycles compared with single-layered 8YSZ TBC samples in TGMF tests. This is because DCL La₂Zr₂O₇ TBC samples had higher residual stress during the thermal cycling process, and their fracture toughness was lower than that of 8YSZ. Bond strength test results showed that 8YSZ TBC samples had higher bond strength compared with La₂Zr₂O₇. The erosion rate of La₂Zr₂O₇ TBC samples was higher than that of 8YSZ samples, due to the lower critical erodent velocity and fracture toughness of La₂Zr₂O₇. DCL porous 8YSZ + La₂Zr₂O₇ had a lower erosion rate than other SCL and DCL La₂Zr₂O₇ coatings, suggesting that porous 8YSZ serves as a stress-relief buffer layer.     

Microstructures and characterization of zirconia-yttria coatings formed in laser and hybrid spray process

Hybrid plasma spraying combined with yttrium-aluminum-garnet laser irradiation was studied to obtain optimum zirconia coatings for thermal barrier use. Zirconia coatings of approximately 150 μm thickness were formed on NiCrAlY bond coated steel substrates both by means of conventional plasma spraying and hybrid plasma spraying under a variety of conditions. Post-laser irradiation was also conducted on the plasma as-sprayed coating. The microstructure of each coating was studied and, for some representative coatings, thermal barrier properties were evaluated by hot erosion and hot oxidation tests. With hybrid spraying, performed under optimum conditions, it was found that a microstructure with appropriate partial densification and without connected porosity was formed and that cracks, which are generally produced in the post-laser irradiation treatment, were completely inhibited. In addition, hybrid spraying formed a smooth coating surface. These microstructural changes resulted in improved coating properties with regard to hardness, high temperature erosion resistance, and oxidation resistance. This paper originally appeared in Thermal Spray: Meeting the Challenges of the 21st Century; Proceedings of the 15th International Thermal Spray Conference, C. Coddet, Ed., ASM International, Materials Park, OH, 1998. This proceedings paper has been extensively reviewed according to the editorial policy of the Journal of Thermal Spray Technology.     

Relationship Between the Microstructure and Thermal Conductivity of Plasma-Sprayed ZrO<Subscript>2</Subscript> Coatings

Plasma-sprayed yttria-stabilized zirconia coatings have a complex microstructure consisting of a variety of pores and cracks. These microstructure features which are determined by the spray process are known to influence the thermal conductivity of coatings. In this article, the microstructure features such as total porosity, large pores, and small pores were quantified by means of scanning electron microscopy (SEM) and image analysis, and for each spray process, the particle velocity and particle temperature were measured prior to impact onto the substrate using the online monitoring system (Spray Watch 2i). Multiple linear regression was used to find the relationship between the particle state and the spray gun parameters. The linear regression models were also investigated between the particle state and the microstructure features, in addition, between the microstructure features and the thermal conductivity. The comprehensive correlation of spray process-microstructure-thermal conductivity was established for plasma-sprayed ZrO₂ coatings.     

Elastic modulus measurements via laser-ultrasonic and knoop indentation techniques in thermally sprayed coatings

Nondestructive techniques for evaluating and characterizing coatings were extensively demanded by the thermal spray community; nonetheless, few results have been produced in practice due to difficulties in analyzing the complex structure of thermal spray coatings. Of particular interest is knowledge of the elastic modulus values and Poisson’s ratios, which are very important when seeking to understand and/or model the mechanical behavior or to develop life prediction models of thermal spray coatings used in various applications (e.g., wear, fatigue, and high temperatures). In the current study, two techniques, laser-ultrasonics and Knoop indentation, were used to determine the elastic modulus of thermal spray coatings. Laser-ultrasonics is a noncontact and nondestructive evaluation method that uses lasers to generate and detect ultrasound. Ultrasonic velocities in a material are directly related to its elastic modulus value. The Knoop indentation technique, which has been widely used as a method for determining elastic modulus values, was used to compare and validate the measurements of the laser-ultrasonic technique. The determination of elastic modulus values via the Knoop indentation technique is based on the measurement of elastic recovery of the dimensions of the Knoop indentation impression. The approach used in the current study was to focus on evaluating the elastic modulus of very uniform, dense, and near-isotropic titania and WC-Co thermal spray coatings using these two techniques. Four different coatings were evaluated: two titania coatings produced by air plasma spray (APS) and high-velocity oxyfuel (HVOF) and two types of WC-Co coatings, conventional and multimodal (nanostructured and microsized particles), deposited by HVOF. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), 5–8 May, 2003, Basil R. Marple and Christian Moreau, Eds., ASM International, 2003.     

Influence of Silver Doping on Photocatalytic Activity of Liquid-Flame-Sprayed-Nanostructured TiO2 Coating

Silver ion was added to liquid feedstock to deposit Ag⁺-doped-nanostructured TiO₂ photocatalytic coatings through liquid-flame spraying. The coating microstructure was characterized by x-ray diffraction (XRD). The photocatalytic performance of coatings was examined by photodegradation of acetaldehyde. The XRD analysis showed that the phase structure of coatings was not significantly influenced by the silver ion doping. However, a shift was found for XRD peaks of anatase TiO₂. The photocatalytic activity of the TiO₂ coatings increased and then decreased with the increase of dopant concentration. The photocatalytic activity of doped coatings was higher than that of pure TiO₂ coating, regardless of the dopant concentration. The enhancement of photocatalytic performance of doped coatings is attributed to co-doping of Ag⁺ ion and metallic Ag. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

In-process exothermic reaction in high-velocity oxyfuel and plasma spraying with SiO<Subscript>2</Subscript>/Ni/Al-Si-Mg composite powder

Reactive thermal spraying, in which thermodynamically stable compounds are formed by expected inprocess reactions, has attracted considerable attention as a result of the wide availability of in situ composite coatings. Such in-process reactions occur differently in high-velocity oxyfuel (HVOF) and plasma spraying because of differences in the flame temperature and speed. In the current study, a composite powder of SiO₂/Ni/Al-Si-Mg was deposited onto an aluminum substrate to fabricate in situ composite coatings by both spraying methods. The coating hardness sprayed with Al-Si-Mg core powder increases with silicon and magnesium content, whereas the coatings by HVOF spraying show higher hardness than those by plasma spraying. In the present reactive spraying, the exothermic reaction of SiO₂ with molten Al-Si-Mg alloy leads to composite materials of MgAl₂O₄, Mg₂Si, and Al-Si matrix. Moreover, a rapid formation of aluminide (NiAl₃), which is introduced by an exothermic reaction of plated nickel with Al-Si-Mg core powder, enhances the reduction of SiO₂, especially in HVOF spraying. A series of in-process reactions proceed mainly during splat layering on a substrate instead of during droplet flight even in plasma spraying. Plasma-sprayed composite coatings become much harder because of the great progress of in-process reactions.     

Properties and rapid consolidation of nanocrystalline 8MoSi<Subscript>2</Subscript>-Si<Subscript>3</Subscript>N<Subscript>4</Subscript> by high frequency induction heated sintering

Nanopowders of MoSi₂ and Si₃N₄ were synthesized from Mo₂N and Si by high ball milling. Dense nanostructured 8MoSi₂-Si₃N₄ composite was consolidated by high frequency induction heated sintering (HFIHS) method within two minutes from mechanically synthesized powders of MoSi₂ and Si₃N₄. Highly dense 8MoSi₂-Si₃N₄ composite with a relative density of up to 97 % was produced under simultaneous application of 80 MPa pressure and the induced current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.