Margaret M. Hyland

Microhardness variation in relation to carbide development in heat treated Cr3C2-NiCr thermal spray coatings

Abstract Cr 3 C 2 –NiCr thermal spray coatings have been extensively used to mitigate high temperature wear. During deposition compositional degradation occurs through dissolution of the carbide phase into the matrix. High temperature exposure leads to transformations in the microstructure, which influences the coating microhardness. While such developments have been investigated in short-term trials, no systematic long-term investigations of the microhardness variation as a function of microstructural development have been presented. In this work, high velocity sprayed Cr 3 C 2 –NiCr coatings were heat treated at 900 °C for up to 60 days in air and argon. With treatment, matrix phase supersaturation was reduced, while widespread carbide nucleation and growth generated an expansive carbide skeletal network. An initial softening of the coatings occurred through matrix phase refinement, the subsequent hardness recovery was a function of carbide development. Treatment in air generated further hardness increases as a result of internal oxidation.

Study of the influence of microstructural properties on the sliding-wear behavior of HVOF and HVAF sprayed WC-cermet coatings

The microstructural properties of WC-Co-Cr and WC-Co coatings deposited by high-velocity oxygen fuel (HVOF) and high-velocity air fuel (HVAF) processes were investigated. The tribological behavior of the coatings was studied by means of pin-on-disk tests. Microcracking of the HVOF sprayed WC-Co coatings did not allow preparation of suitable disks for wear tests. The wear rates of the remaining coatings were determined, and wear tracks on the coatings and counterbodies were investigated by SEM. The HVAF sprayed coatings showed greater sliding-wear resistance compared to the HVOF coatings. The prime wear mechanism in the WC-Co HVAF coatings was adhesive wear. The cobalt matrix is lubricious, resulting in very low wear rates and low debris generation. The main wear mechanisms in the WC-Co-Cr coatings were adhesive and abrasive wear. Adhesive wear results in coating material dislodgments (i.e., “pullouts”) that become trapped in the contact zone and act as a third-body abrasive. Particle pullout from the coating significantly increases the wear rate of the coated specimen. The HVAF/WC-Co-Cr coatings exhibited better resistance to particle pullout, resulting in a considerably lower wear rate than the HVOF/WC-Co-Cr coatings.

Comparative study of WC-cermet coatings sprayed via the HVOF and the HVAF process

The high velocity air fuel (HVAF) system is a high-velocity combustion process that uses compressed air and kerosene for combustion. Two WC-cermet powders were sprayed by the HVAF and the high-velocity oxyfuel (HVOF) processes, using an AeroSpray gun (Browning Thermal Systems Inc., Enfield, New Hampshire) and a CDS-100 gun (Sulzer Plasma Technik, Wohlen, Switzerland) respectively. Several techniques, including x-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy, were used to characterize the microstructures and phase distribution of the powders and coatings. In addition, mechanical properties such as hardness and wear resistance (pin-on-disk) were investigated. A substantial amount of W2C was found in the HVOF coatings, as well as a high concentration of tungsten in the binder phase, indicating that oxidation and dissolution processes change the composition and microstructure from powder to coating during spraying. This was in contrast to the HVAF coatings in which composition and microstructure were unchanged from that of the powder. Additionally, the wear resistance of the HVAF coatings was superior to that of the HVOF coatings.

Surface and charge transport characterization of polyaniline-cellulose acetate composite membranes.

This study elucidates the charge transport processes of polyaniline (PANI) composite membranes and correlates them to the PANI deposition site and the extent of PANI surface layering on the base microporous membranes. PANI was deposited either as a surface layer or inside the pores of cellulose acetate microporous membranes using various in situ chemical polymerization techniques. The extent of PANI layering at the surface of the base membrane and its oxidation and doping states were characterized using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). PANI deposition on the membranes showed a strong dependence on the polymerization technique and polymerization time within a single technique. In XPS, the deconvolution of C 1s and N 1s core-level spectra of the composite membranes was used to quantify the extent of PANI layering at the surface along with its oxidation and doping states. PANI incompletely covered the surface of the base microporous membranes for all the employed techniques. However, the extent of the layering increased with the polymerization time in a particular technique. The charge transport through the bulk membrane and charge transfer at the membrane/electrode interface were studied by electrochemical impedance spectroscopy (EIS). The data were analyzed using the equivalent circuit modeling technique. The modeling parameters revealed that PANI deposition at the surface enhanced the interfacial charge transfer but the process depended on the extent of the surface coverage of the membrane. In addition, the charge transport in the bulk membrane depended on the PANI intercalation level, which varied depending on the polymerization technique employed. In addition, the EIS of electrolyte-soaked membranes was also conducted to evaluate the effects of PANI deposition site on charge transport in the presence of an electrolyte. PANI layering at the pore walls of the base membrane from diaphragmatic polymerization in a two-compartment cell showed that charge transport processes were strongly affected by the interaction of the electrolyte with the PANI layer at the pore surface. This study successfully showed the dependence of charge transport mechanisms of PANI composite membranes on the PANI deposition site and extent of surface layering at the membrane surface.

Long-term carbide development in high-velocity oxygen fuel/high-velocity air fuel Cr3C2-NiCr coatings heat treated at 900 °C

During the deposition of Cr3C2-NiCr coatings, compositional degradation occurs, primarily through the dissolution of the carbide phase into the matrix. Exposure at an elevated temperature leads to transformations in the compositional distribution and microstructure. While these have been investigated in short-term trials, no systematic investigations of the long-term microstructural development have been presented for high-velocity sprayed coatings. In this work, high-velocity air fuel (HVAF) and high-velocity oxygen fuel (HVOF) coatings were treated at 900 °C for up to 60 days. Rapid refinement of the supersaturated matrix phase occurred, with the degree of matrix phase alloying continuing to decrease over the following 20 to 40 days. Carbide nucleation in the HVAF coatings occurred preferentially on the retained carbide grains, while that in the HVOF coatings developed in the regions of greatest carbide dissolution. This difference resulted in a variation in carbide morphologies. Preferential horizontal growth was evident in both coatings over the first 20 to 30 days of exposure, beyond which spheroidization of the microstructure occurred. After 30 days, the carbide morphology of both coatings was comparable, tending toward an expansive structure of coalesced carbide grains. The development of the carbide phase played a significant role in the microhardness variation of these coatings with time.

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.

Effect of substrate hardness on splat morphology in high-velocity thermal spray coatings

In this study, Ni-chrome alloy particles were thermally sprayed onto a variety of substrate materials using the high-velocity air fuel (HVAF) technique. Although the various substrate materials were sprayed using identical powder material and thermal spray conditions, the type and variation of splat morphologies were strongly dependent on the substrate material. Predominantly solid splats are observed penetrating deeply into softer substrates, such as aluminum, whereas molten splats were observed on harder substrates, which resisted particle penetration. The observed correlation between molten splats and substrate hardness could be due a dependency of deposition efficiencies of solid and molten splats on the substrate material. However, it was found that conversion of particle kinetic energy into plastic deformation and heat, dependent on substrate hardness, can make a significant contribution towards explaining the observed behavior.

Control of Polyaniline Deposition on Microporous Cellulose Ester Membranes by in Situ Chemical Polymerization

Polyaniline (PANI) can be deposited either on the surface or in the bulk of a microporous membrane by various chemical oxidative polymerization techniques. Each technique has distinctive effects on the PANI site and extent of deposition on the base membrane. In the present study, mixed cellulose ester (ME) membranes with tortuous pore morphology were used as base membranes. The chemical oxidative polymerization techniques employed, included polymerization using an in-house-built two-compartment permeation cell. The resultant composite membranes have been characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR-ATR), and electrical conductivity measurements. The results showed that PANI was layered on the pore walls of the membrane using two-compartment permeation cell. Vapor-phase polymerization yielded a surface layer of PANI with little deposition in the bulk. A distorted PANI surface layer was achieved by solution-phase (dip) polymerization. Moreover, asymmetric PANI deposition within the membrane bulk was evidenced using two-compartment permeation cell. Composite membranes synthesized using two-compartment cell showed highest levels of conductivity (approximately 10(-2) S/cm) as compared to the membranes modified by single-step solution-phase polymerization. FTIR-ATR results indicated the extent of PANI coating and its oxidation state which was identified as doped emeraldine PANI, from all the employed techniques. Asymmetric deposition and extent have been explained in terms of the physical and chemical reaction steps involved in the heterogeneous aniline polymerization reactions in the two-compartment cell technique.

Role of oxides in high velocity thermal spray coatings

Abstract Thermal spray coatings applied with high velocity techniques such as high velocity air fuel (HVAF), produce coatings with superior quality in comparison to other traditional techniques such as plasma spraying. To date, our knowledge of the bonding processes and the structure of the particles within thermal spray coatings is very subjective. To improve our understanding of these materials, especially of the surface oxide layer, Ni80/Cr20 HVAF thermally sprayed coatings were studied with scanning electron microscope, nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS). In particular, NRA and XPS were used to characterise the oxide composition that gives the coatings their excellent oxidation resistance. The surface oxide on the Ni80/Cr20 particles was found to be only 7 nm thick and enriched in SiO 2 . A surprising finding was that the composition of the Ni80/Cr20 powder remained unchanged during the coating process despite the high velocity application with the HVAF method.

Evidence of mechanical interlocking of NiCr particles thermally sprayed onto Al substrates

Ni-chrome alloy particles were thermally sprayed onto aluminum substrates using the high-velocity air fuel technique. The particle substrate interface was investigated with focused ion beam microscopy, cross-sectional scanning electron microscopy, and cross-sectional transmission electron microscopy. No evidence of melting or chemical bonding was found in the samples. Instead, evidence of mechanical bonding was found that had been predicted by a previous theoretical study by Grujicic et al. At locations where the particle and substrate are in intimate contact, the interface exhibited interlocking features. These features are caused by the effects of turbulence due to interfacial instability and mixing at the interface during the coating process, resulting in a strong particle-substrate bond. Conversely, separated interfaces exhibited smooth surfaces, suggesting insignificant bonding between the particle and the substrate. The discovery of these interfacial formations, together with no evidence of chemical bonding across the particle-substrate interface indicate that mechanical interlocking is the dominant bonding mechanism.