Timothy J. Eden

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Investigation and characterization of Cr3C2-based wear-resistant coatings applied by the cold spray process

The purpose of this study was to explore the potential of the cold spray (CS) process in applying Cr3C2-25wt.%NiCr and Cr3C2-25wt%Ni coatings on 4140 alloy for wear-resistant applications. This article discusses the improvements in Cr3C2-based coating properties and microstructure through changes in nozzle design, powder characteristics stand off distance, powder feed rate, and traverse speed that resulted in an improved average Vickers hardness number comparable to some thermal spray processes. Cold spray process optimization of the Cr3C2-based coatings resulted in increased hardness and improved wear characteristics with lower friction coefficients. The improvement in hardness is directly associated with higher particle velocities and increased densities of the Cr3C2-based coatings deposited on 4140 alloy at ambient temperature. Selective coatings were evaluated using x-ray diffraction for phase analysis, optical microscopy (OM). and scanning electron microscopy (SEM) for microstructural evaluation, and ball-on-disk tribology experiments for friction coefficient and wear determination. The presented results strongly suggest that cold, spray is a versatile coating technique capable of tailoring the hardness of Cr3C2-based wear-resistant coatings on temperature sensitive substrates.

Amorphous phase formation in spray deposited AlYNiCo and AlYNiCoZr alloys

Al 85 Y 8 Ni 5 Co 2 and Al 84 Y 3 Ni 8 Co 4 Zr 1 (at%) alloy billets were prepared by spray forming. Two ratios of volumetric gas flow rate to mass of metal flow rate (G/M) were used for the Al 85 Y 8 Ni 5 Co 2 alloy (G/M 5 6.4 m 3 /kg and 10.0m 3 /kg) and one G/M (8.7 m 3 /kg) was used for the Al 84 Y 3 Ni 8 Co 4 Zr 1 alloy. The superheat temperature was 1170K and the diameter of the nozzle bore of the atomizer used was 4.4 mm. The molten metal was sprayed at a rate of 3.1 kg/min and nitrogen gas was used for atomization. The resulting billets weighed about 7.0kg and 2.1kg for the Al 85 Y 8 Ni 5 Co 2 and Al 84 Y 3 Ni 8 Co 4 Zr 1 alloys, respectively. The overspray powder was collected in a cyclone separator and accounted for about 19% and 30% of the starting charge for the Al 85 Y 8 Ni 5 Co 2 and the Al 84 Y 3 Ni 8 Co 4 Zr 1 alloy, respectively. The billets and the powder were analyzed by X-ray diffraction using Cu K a radiation (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) system, electron probe microanalysis (EPMA) and transmission electron micros

Self-Lubricating Cold-Sprayed Coatings Utilizing Microscale Nickel-Encapsulated Hexagonal Boron Nitride

It is often beneficial to modify surfaces to gain desirable properties such as improved wear and friction resistance. Self-lubricating coatings can improve the performance of contacting surfaces and extend component lifetimes by reducing the coefficient of friction and/or improving resistance to specific wear modes. With these goals in mind, self-lubricating coatings of hexagonal boron nitride (hBN) particles in a deposited nickel matrix were investigated and optimized for friction and wear. These self-lubricating coatings were created via high-velocity particle consolidation or cold spray using micrometer-sized hBN powder encapsulated by nickel and nickel phosphorous alloys. Relatively thick nickel encapsulation via electrolesss Ni plating was required to aid in coating bonding/formation by “tricking” the hBN into acting as monolithic Ni during deposition. Once deposited on aluminum substrates, the coatings were analyzed and found to exhibit enhanced mechanical and tribological properties such as high bond strength and microhardness, a relatively low coefficient of friction, and improved reciprocating wear behavior relative to pure cold-sprayed Ni coatings. Furthermore, the encapsulation process was found to be both scalable and amenable to relatively small hBN particles.

Cold-Sprayed Ni-hBN Self-Lubricating Coatings

Feedstock preparation strategies were explored to produce composite admixed, milled, and precoated (encapsulated) powders of nickel–hexagonal boron nitride (Ni-hBN) for cold-sprayed self-lubricating coatings. The resulting cold-sprayed coatings were then examined for microstructural homogeneity and composition, as well as bond strength, microhardness, and relevant wear behaviors. Though admixed powders were easy to prepare and economical, milled and precoated formulations provided the advantage of aiding contact between Ni and lubricant powders prior to spraying that ultimately improved deposition and properties. The maximum amount of hBN that could be effectively built into the cold-sprayed Ni coatings was approximately 6 wt%. Results of the study also indicated that the composite coatings exhibited slightly higher hardness and reduced adhesive strength relative to a baseline of pure Ni layers. Moreover, some reductions in friction and expected decreases in bond strength and lubricant uniformity were observed when more than 4 wt% of lubricant was retained in the coatings. Given these findings, the most promising path to improve the amount, uniformity, and influence of the lubricant may be to encapsulate smaller particles with thicker levels of Ni to “trick” the composite particle to bond as pure Ni.

Evaluation of Ice-Adhesion Strength on Erosion-Resistant Materials

Ice-adhesion properties were evaluated for coating systems based on titanium nitride applied via cathodic-arc physical vapor deposition developed for rotorcraft erosion caps. The ice-adhesion strength of titanium nitride and titanium aluminum nitride was determined experimentally and compared to the ice-adhesion strength of uncoated metallic materials currently used on rotor-blade leading-edge caps: stainless steel 430, Inconel 625, and titanium grade 2. Environmental and material parameters were investigated to identify which were most influential on impact ice-adhesion strength. The effects of median volumetric diameter of the cloud droplets, liquid water content of the cloud, ambient temperature, surface roughness, and material grain direction were tested on stainless steel 430. Tests revealed that surface roughness and temperature have the greatest effect on ice-adhesion strength. There was an increase in adhesion strength of 670% from −8 to −16 °C and 250% increase from 0.61 to 2.67Ra  μm. An increas...

Ice Adhesion Mechanisms of Erosion-Resistant Coatings

The physical mechanism responsible for impact ice adhesion variations for different coatings is not well understood. This research examines the effects of surface characteristics on ice adhesion strength for three erosion-resistant materials. The materials tested were titanium grade 2, titanium aluminum nitride coated on titanium grade 2, and titanium nitride coated on titanium grade 2. The surface roughness of the material contributed to the ice adhesion strength but did not explain the variation in ice adhesion strength for materials at similar surface roughness values. To compare the materials, the ice adhesion strength was divided by the environmental temperature to allow for the comparison of ice adhesion measured at varying temperature conditions. The new quantity was called temperature-adjusted adhesion strength. When the surface roughness of the titanium grade 2 substrate increased from 26.4 to 86.1  μin. Ra, the temperature-adjusted adhesion strength increased 29%. The titanium nitride temperatur...

Fragmentation of a Steel Ring under Explosive Loading

There is a great deal of interest in the behavior of metallic materials under high strain rate loading. Finite Element Analysis (FEA) could be used to model these materials with a reduction in the amount of experimentation needed for characterization. A finite element model of a metallic ring under high strain rate loading was developed using the Johnson-Cook failure model in Abaqus Computer Aided Engineering (CAE). The ring was modeled both axisymmetrically and in three dimensions. Failure was determined by defining a failure initiation value to start the process of element deletion. It was found that element deletion would occur when the failure strain initiation value was less than 1x10−4. Results of both axisymmetric and 3-D were found to be within 3% of each other with respect to maximum von Mises stress, and failure modes were identical. The effects of model changes and loading conditions are investigated.