HVOF Spraying Research Articles

The Effect of Oxygen Flow Rate in HVOF Spraying on Oxidation Behavior of Cr<SUB><B>3</B></SUB>C<SUB><B>2</B></SUB> (7 mass%NiCr) Coatings

This study was performed to investigate the influence of the oxygen flow rate in HVOF spraying on the oxidation behavior of Cr 3 C 2 (7 mass%NiCr) coatings. The oxide was formed on the coatings during HVOF spraying regardless of the oxygen flow rate. The oxide observed at the coatings was Cr 2 O 3 , but chromium oxides of other forms were not observed. Cr 3 C 2 (7 mass%NiCr) coatings were stable in a temperature range up to 1073 K for 50 h. However, the oxidation behavior of the coatings at 1273 K showed great difference from that at temperature up to 1073 K and the oxygen flow rate had a significant influence on the oxidation behavior of the coatings. The surface of the coatings sprayed with 229 and 243 I/min-oxygen flow rate became porous by the aggressive evaporation of CO, CO 2 gases and the oxide clusters were partially formed after isothermal oxidation at 1273 K for 50h. The oxide clusters were complex oxides consisted of Ni and Cr, whereas the porous surface was composed of only Cr 2 O 3 . TG analysis revealed that the mass loss by intensive evaporation of gaseous oxide, in the case of the coatings sprayed with 229 I/min-oxygen flow rate, began after the oxidation time of approximately 42 h at 1273 K. The apparent activation energy (Q) for oxidation varied from 89.04 to 99.96 kJ mol -1 with increasing oxygen flow rate.

206 高速フレーム溶射により形成させたWC-Co皮膜のはく離強度に及ぼす繰返し接触圧力の影響(機械材料I)

206 高速フレーム溶射により形成させたWC-Co皮膜のはく離強度に及ぼす繰返し接触圧力の影響(機械材料I)

The Influence of Fuel/Oxygen Ratio in HVOF Spraying on the Deposition of Cr<SUB>3</SUB>C<SUB>2</SUB> Coatings

This study was performed to investigate the influence of fuel/oxygen ratio (F/O = 3.2, 3.0, 2.8) on the characteristics of the HVOF sprayed Cr3C2 (7 mass%NiCr) coatings after heat treatment up to 1000°C. Decomposition and oxidation of the Cr3C2 occurred during spraying. The degree of decomposition from Cr3C2 to Cr7C3 was increased with decreasing the F/O ratio. The micro-structural differences of the as-sprayed coatings with F/O ratio cannot be distinguished. However, large pores were diminished and then the coatings became dense by heat treatment. Micro-hardness of the as-sprayed coating which is sprayed with F/O = 3.2 condition was the highest (Hv300=1140) and the hardness was increased to 1660 after heat treatment at 1000°C for 50 h in air. It was supposed that hardness was increased due to the formation of Cr2O3 within the Cr3C2/Cr7C3 matrix and the densification of the coating layer during heat treatment. Mass loss of the as-sprayed coating by erosion varied from 79 to 80.8 mg depending on F/O ratio and varied from 47.2 to 49 mg after heat treatment at 600°C. Erosion resistance of the coating was improved about 40% after heat treatment at 600°C. It is concluded that improvement of erosion resistance of the coatings was due to production of the coatings with a very dense microstructure, high cohesive strength and a very high hardness value by heat treatment.

Heat transfer during the formation of an HVOF sprayed WC–Co coating on a copper substrate

Mathematical modelling of the heat transfer between a WC–Co coating and a copper substrate during HVOF spraying is undertaken. This modelling includes the investigation of temperature variation, coating solidification, melting and solidification in the substrate interfacial region, and specific features of the substrate–coating thermal interaction. The results obtained agree well with experimental data.

Microstructure and Properties of Tungsten Carbide Coatings Sprayed with Various High-Velocity Oxygen Fuel Spray Systems

This article reports on a series of experiments with various high-velocity oxygen fuel spray systems (Jet Kote, Top Gun, Diamond Jet (DJ) Standard, DJ 2600 and 2700, JP-5000, Top Gun-K) using different WC-Co and WC-Co-Cr powders. The microstructure and phase composition of powders and coatings were analyzed by optical and scanning electron microscopy and x-ray diffraction. Carbon and oxygen content of the coatings were determined to study the decarburization and oxidation of the material during the spray process. Coatings were also characterized by their hardness, bond strength, abrasive wear, and corrosion resistance. The results demonstrate that the powders exhibit various degrees of phase transformation during the spray process depending on type of powder, spray system, and spray parameters. Within a relatively wide range, the extent of phase transformation has only little effect on coating properties. Therefore, coatings of high hardness and wear resistance can be produced with all HVOF spray systems when the proper spray powder and process parameters are chosen.

Laser Shock Processing of Al-SiC Composite Coatings

Laser shock processing (LSP) is a technique of surface treatment (similar to shot peening) in which laser-induced mechanical shocks develop compressive stresses in the material. The stresses are of sufficient intensity to modify microstructure and properties of the coatings. In the present study, laser shocks of power density of 5 to 8 GW/cm2 power density, generated by means of a neodymium-glass laser, were used to treat Al + SiC composite coatings deposited by means of a HVOF spraying technique. The laser processed samples were metallographically prepared, and their microstructure was investigated by optical microscope and SEM. The latter was also used to investigate the surface morphology of the laser treated specimens. Finally, the microhardness and oscillating wear resistance of the coatings were tested and compared to data obtained for as-sprayed samples.

Metallurgical and Metrological Examinations of the Cylinder Liner-Piston Ring Surfaces After Heavy Duty Diesel Engine Testing©

This paper will discuss results of metallurgical and metrological examinations of cylinder liner and piston ring surfaces before and after 150, 1000 and 4000 hours of heavy duty diesel engine testing. Three different composition coatings and three methods of application were investigated, including electroplating, plasma and HVOF spraying. To examine metallurgical parameters such as porosity, microhardness, microstructure and their changes after testing, scanning electron microscopy and energy dispersive X-ray spectroscopy were used. The cylinder liner and piston ring surface profile and roughness parameters include Ra, Rz, Rk, Rvk, Rvk, skewness, oil retention volume, etc. The mechanisms of scuffing origin and final wear were evaluated for different piston ring coatings and methods of application. The paper will show the correlation between metallurgical and metrological parameters and efficiency of the cylinder liner-piston rings tribological system. The data from these examinations formulates required performance for transformation of abnormal and/or severe type of wear to mild processes. Presented as a Society of Tribologists and Lubrication Engineers paper at the World Tribology Congress in London, United Kingdom, September 8–12, 1997

Thermal spray processing of nanoscale materials—A conference report with extended abstract

Significant interest has been generated recently in the field of nanoscale materials. This interest stems not only from the outstanding properties that can be obtained in such materials but also from the realization that early skepticism about the ability to produce high-quality, unagglomerated nanoscale powder was unfounded. There are many methods available to form nanocrystalline materials that can be further processed to evolve nanophase and or nanocrystalline structures. Some of these materials are becoming fully commercialized and, accordingly, the focus is shifting from synthesis to processing; that is, how to make useful coatings and structures from these powders. The potential applications span the whole spectrum of technology, for example, from thermal barrier coatings for turbine blades to wear-resistant rotating parts. Significant progress has been made in various aspects of processing nanoscale materials. Most of this work is focused on the fabrication of bulk structures. However, the process most likely to have the earliest (and perhaps the greatest) major technological impact is deposition of coatings by thermally activated processes. This includes thermal spray methods such as HVOF and plasma spray, but also includes innovations such as chemical vapor condensation (CVC) and a number of exciting new combustion processes. The objective of this conference was to assess the state -ofthe-art in understanding the science and technology of thermally sprayed nanocrystalline coatings. The objective was accomplished by addressing the synergism between processing, physical and mechanical characteristics, and the behavior of these

Effect of the substrate and of thermophoresis on the acceleration and heating of particles during HVOF spraying

A theoretical model is used to evaluate the velocity and temperature of a spherical particle during its flight in a HVOF thermal spraying system. The computations are performed for WC–Co particles. Computations show that it is necessary to take into account the presence of the substrate on the flow of the exhaust gases. This effect is important because of the existence of a boundary-layer zone where the velocity of the gases is ultimately zero at the substrate surface. Moreover, thermal forces or thermophoresis due to the temperature gradient in the spraying direction are also shown to have a significant effect on the evolution of the velocity and temperature of the particle for small particle sizes.

Modelizacion de la formación de recubrimientos de WC-Co por proyección HVOF sobre sustratos de cobre

Present paper deals with the mathematical simulation of the heat transfer between a WC-Co coating and a copper substrate during HVOF spraying. This modelling includes the investigation of temperature variation, coating solidification, melting and subsequent solidification in the substrate interfacial region and specific features of the substrate-coating thermal interaction. The results obtained are used for modelling of the development of the coating structure and adhesion during HVOF spraying of the WC-Co powder on a copper substrate. Two types of substrate were considered: smooth (polished) and rough. Variations of solidification times, solidification velocity, thermal gradient and cooling velocity in the coating and substrate interfacial region are studied. Development of the amorphous and crystalline structures in the coating and of the crystalline structure in the substrate interfacial region is discussed. Behaviour of the crystal size and intercrystalline distance with respect to the thermal spray parameters and morphology of the substrate surface is analyzed. Optimal conditions for the formation of fine and dense crystalline structure are determined. Structural changes in the solid state of the substrate occurring because of heating and rapid cooling are considered. Mechanical and thermal mechanisms of development of the substrate-coating adhesion are discussed. Results obtained agree well with experimental data.

Impact wear performances of Cr 3C 2NiCr coatings by plasma and HVOF spraying

Three kinds of Cr 3C 225%NiCr coatings prepared by APS-Ar H 2 , APS-Ar He and HVOF spraying, respectively, have been studied with a self-made impact wear rig. The wear resistant performances of Cr 3C 225%NiCr coatings were tested under different impact stress. The wear losses and worn surface profiles are measured, and the corresponding wear mechanisms were studied with Scanning Electronic Microscope (SEM). It is found that wear resistance of Cr 3C 225%NiCr coatings under different impact conditions increases in the order of APS-Ar H 2 APS-Ar He and HVOF spraying. The HVOF sprayed Cr 3C 225%NiCr coatings show the best impact wear resistance due to the denser structure and fewer defects. The wear mechanisms of Cr 3C 225%NiCr coatings under impact condition is impact fatigue, which consists of two wear processes of plastic smearing and breaking-off. The plastic smearing produces plastic deformation of the thin layer in the outer coatings and then causes microcracks in the near subsurface due to stress accumulation. There are horizontal and perpendicular microcracks. The former commonly nucleate from the boundary of lamellae, while the latter may occur from the sub-microcracks inside lamellae. The propagating and merging of both horizontal and perpendicular microcracks finally give rise to the breaking-off of coatings. The cohesive defects such as stratification, porosity and microcracks in the lamellae have much effects on the wear resistance of coatings.

Three dimensional simulation of HVOF spray deposition of nanoscale materials

We have developed and validated three dimensional simulation capability to model the TS systems' gas and coating powder flow for the TS process analysis, and to illustrate the roots of the TS systems' inefficiencies. The same capability can be used to design optimized TS systems, and to optimize and control the coating process. Examples are given for TS process designs with improved performance and system efficiency. The use of numerical simulation will be especially crucial for plating with nanoscale powders. We will discuss specific equipment and process solutions that will make nanoscale powers coating a viable industrial process.

Dynamic processes during high velocity oxyfuel spraying

High velocity oxyfuel spraying is a new, rapidly developing technology for coating formation which is now challenging plasma spraying. Dynamic processes during HVOF spraying have considerable influence on the coating quality. The dynamic processes considered are as follows: combustion and gas dynamics, mechanical and thermal inflight behaviour of the powder particles, and droplet flattening. The corresponding literature is also reviewed. The following points are taken into account: fluid behaviour inside and outside a spraying gun; fluid-particle interactions; particle shape and morphology; internal heat conduction in powder particles; mass transfer (dissolution, diffusion, etc.) inside the composite particles; particle heating, fusion, cooling, and solidification; surface roughness, splat solidification, and mass loss of the droplet liquid phase during droplet flattening. Good agreement between theoretical and experimental results is shown to exist. The results obtained can be used for prediction of the optimum conditions of spraying.

Ｎｉ‐Ａｌ系メカニカルアロイ粉末のＤＣプラズマ溶射による金属間化合物皮膜の作製

Experimental conditions for the fabrication of intermetallic compound coatings by DC plasma spraying of Ni/Al mechanically alloyed powders have been investigated.The results obtained are summarized as follows ;(1) In atmospheric plasma spraying, APS, the obtained coating was composed of two or more phases : Ni-rich phase, and Alrich phase. Ni/Al MA powder seemed to be subjected to phase separation during plasma spraying. On the other hand, homogeneous intermetallic coatings were obtained by low pressure plasma spraying, LPPS, due to the diffusion of two element in the sprayed powders.(2) The dominating factor for phase separation for MA powders during plasma spraying seemed to be the heat flow from surrounding plasma ; the contributions of other factors, such as, mixing of the powder, composition of working gas, oxidation of the powder and acceleration force in plasma flame, were negligible.(3) By low pressure plasma spraying or HVOF spraying of MA powder, the intermetallic coating can be fabricated easily. Even in the APS process, intermetallic coating can be obtained by the spraying of sizeable MA powder.

Interacción térmica recubrimiento-sustrato en la proyección a alta velocidad (HVOF) de partículas (polvo) de WC-Co

The mathematical simulation of the thermal interaction between a 34CrMo4 (UNS-G41350) steel substrate and a coating formed by the droplets of WC-12 % Co powder particles during HVOF spraying is undertaken. Analysis of the heat transfer processes permitted the investigation of the temperature evolution, coating solidification, substrate fusion and solidification, particular features of the thermal interactions between the substrate and the coating as well as between the successive coating layers. The analysis has also permitted to estimate the optimal conditions of the substrate and the coating structure formation. The obtained results were used in subsequent articles to predict the structure parameters, which agree with the experimental data.

Study of interface interactions for metal-metal and metal-ceramic coatings obtained by plasma and HVOF spraying

Study of interface interactions for metal-metal and metal-ceramic coatings obtained by plasma and HVOF spraying

Thermal interaction between WC-Co coating and steel substrate in process of HVOF spraying

The WC-Co powders can be used to produce good adhesive and wear resistant HVOF thermal spray coatings on steel and light alloys substrates. In order to understand the properties of this kind of coating, the phases which are present in the coatings and structure changes during post heat treatments have been investigated. Although the coating properties depend very much on the structure developed in the substrate-coating interfacial region it has not been yet investigated in detail. The present study is devoted to the experimental and theoretical analysis of this interfacial region. The structure characterization has been performed mainly through the use of transmission electron microscopy. To provide a theoretical investigation a realistic prediction model of the process has been developed and on its base the mathematical simulation of the substrate-coating thermal interaction has been undertaken.

A prediction model to assist plasma and HVOF spraying

A theoretical prediction model is developed to estimate the in-flight velocity, temperature and size of a particle during plasma and HVOF spraying. The salient features of this model are outlined and the model predictions are examined through some example calculations performed under typical spraying conditions. Comparison with experimental data reveals that the present method enables accurate predictions. The principal advantages and shortcomings of the plasma and HVOF systems are also discussed based on the model estimates.