Douglas E. Wolfe

Synthesis of titanium carbide/chromium carbide multilayers by the co-evaporation of multiple ingots by electron beam physical vapor deposition

Abstract Titanium carbide and chromium carbide multilayer coatings with varying individual layer thicknesses were synthesized by the co-evaporation of titanium, chromium, and carbon (through tungsten) ingots by electron beam-physical vapor deposition. The adhesion of the multilayer coatings was found to be greater than 50 N. The hardness of the titanium carbide/chromium carbide multilayer coatings was found to increase from 1302 VHN 0.050 to 2052 VHN 0.050 by decreasing the thickness of the individual layer from 1.2 to 0.1 μm. In addition, the average grain diameter was also found to decrease from 3.315 to 0.356 μm by decreasing the thickness of the individual layers. The fracture toughness of the TiC/CrC multilayer coatings decreased from 4.179 to 1.411 MPa-m 1 2 with decreasing layer thickness. Lastly, the amount of compressive stress in both the TiC and CrC layers within the multilayer coating was found to decrease with decreasing individual layer thickness. The samples were characterized by various techniques including Vickers hardness, X-ray diffraction, scanning electron microscopy, scratch testing and fracture toughness, with the results being presented.

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.

Synthesis and characterization of multilayered TiC/TiB2 coatings deposited by ion beam assisted, electron beam–physical vapor deposition (EB–PVD)

Abstract Polycrystalline 4 μm and 14 μm thick multilayer TiB2/TiC coatings with varying number of total layers (2–20) were deposited by ion beam assisted, electron beam–physical vapor deposition (IBA, EB–PVD) on WC–6wt.%Co–0.3wt.%TaC substrates. The average Vickers hardness numbers were found to increase with increasing total number of layers and ranged from 3215 VHN0.050 to 3726 VHN0.050 and 3294 VHN0.050 to 3991 VHN0.050 for the 4 μm and 14 μm thick multilayer TiB2/TiC coatings, respectively. The adhesion of the multilayer coatings was found to be greater than 50 N for a 4 μm thick film and less than 30 N for the thicker films. The degree of crystallographic texture was found to change with varying total number of layers, as well as the orientation of both materials within the multilayer system. The grain size and amount of residual compressive stress was found to decrease with increasing number of layers. In addition, the fracture toughness was found to decrease with increasing total number of layers. This paper discusses the effect of changing the individual layer thickness (i.e. total number of layers) for TiC/TiB2 multilayer coatings deposited on WC–6wt.%Co–0.3wt.%TaC cutting inserts on the average Vickers hardness number, adhesion, stress, fracture toughness, argon impurities, average crystallite size, crystallographic texture, surface morphology and microstructure using a variety of characterization techniques.

Architecture of thermal barrier coatings produced by electron beam-physical vapor deposition (EB-PVD)

Extremely high temperatures and severe atmospheric conditions in the hot section of aircraft engines during operation result in degradation and structural failures of turbine components. Replacing these components is very expensive. Thermal barrier coatings (TBC) composed of ZrO2-8wt%Y2O3(8YSZ) applied by Electron Beam-Physical Vapor Deposition (EB-PVD) to turbine components offer excellent properties for thermal protection and resistance against oxidation - induced erosion and corrosion. However, the life of turbine components is still limited due to premature failure of the TBC. It is hypothesized that the life of the coated components can be extended by lowering the thermal conductivity of the TBC by creating multiple non-distinct or distinct interfaces and alloy additions such as Nb-oxide which will result in a reduction in the thermal conductivity and oxygen transport through the coating. This paper presents the microstructural results of standard 8YSZ, layered 8YSZ, Nb-oxide alloyed 8YSZ and functionally graded 8YSZ with Nb-oxide deposited by EB-PVD. TBC samples were examined by various methods including scanning electron microscopy (SEM), high-resolution optical microscopy (OM), X-ray diffraction (XRD), and thermal cycling tests. The preliminary results strongly suggest that multiple interfaced TBC exhibits better oxidation resistant properties as compared to standard and alloyed TBC.

Functionally gradient ceramic/metallic coatings for gas turbine components by high-energy beams for high-temperature applications

Failure of turbine blades generally results from high-temperature oxidation, corrosion, erosion, or combinations of these procedures at the tip, and the leading and trailing edges of a turbine blade. To overcome these limitations, functionally gradient ceramic/metallic coatings have been produced by high-energy beams for high-temperature applications in the aerospace and turbine industries to increase the life of turbine components. Thermal spray processes have long been used to apply high-temperature thermal barrier coatings to improve the life of turbine components. However, these processes have not met the increased demand by the aerospace and turbine industries to obtain higher engine temperatures and increased life enhancement as a result of the inhomogeneous microstructure, unmelted particles, voids, and poor bonding with the substrate. High-energy beams, i.e. electron beam-physical vapour deposition (EB-PVD), laser glazing, laser surface alloying, and laser surface cladding, have been explored to enhance the life of turbine components and overcome the limitations of the thermal spray processes. EB-PVD has overcome some of the disadvantages of the thermal spray processes and has increased the life of turbine components by a factor of two as a result of the columnar microstructure in the thermal barrier coating (TBC). Laser glazing has been used to produce metastable phases, amorphous material, and a fine-grained microstructure, resulting in improved surface properties such as fatigue, wear, and corrosion resistance at elevated temperatures without changing the composition of the surface material. Laser surface alloying and laser surface cladding have shown promising results in improving the chemical, physical, and mechanical properties of the substrates surface. Metal-matrix composite coatings have also been produced by a laser technique which resulted in increased wear and oxidation-resistant properties. The advantages and disadvantages of thermal spray processes, EB-PVD, laser glazing, laser surface alloying, and laser surface cladding will be discussed. Microstructural evolution of thermal barrier coatings, recent advancements in functionally gradient coatings, laser grooving, and multilayered textured coatings will also be discussed.

Subnanoradian X-ray phase-contrast imaging using a far-field interferometer of nanometric phase gratings

Hard X-ray phase-contrast imaging characterizes the electron density distribution in an object without the need for radiation absorption. The power of phase contrast to resolve subtle changes, such as those in soft tissue structures, lies in its ability to detect minute refractive bending of X-rays. Here we report a far-field, two-arm interferometer based on the new nanometric phase gratings, which can detect X-ray refraction with subnanoradian sensitivity, and at the same time overcomes the fundamental limitation of ultra-narrow bandwidths (Δλ/λ~10−4) of the current, most sensitive methods based on crystal interferometers. On a 1.5% bandwidth synchrotron source, we demonstrate clear visualization of blood vessels in unstained mouse organs in simple projection views, with over an order-of-magnitude higher phase contrast than current near-field grating interferometers.

Nanostructured component fabrication by electron beam-physical vapor deposition

Fabrication of cost-effective, nano-grained net-shaped components has brought considerable interest to Department of Defense, National Aeronautics and Space Administration, and Department of Energy. The objective of this paper is to demonstrate the versatility of electron beam-physical vapor deposition (EB-PVD) technology in engineering new nanostructured materials with controlled microstructure and microchemistry in the form of coatings and net-shaped components for many applications including the space, turbine, optical, biomedical, and auto industries. Coatings are often applied on components to extent their performance and life under severe environmental conditions including thermal, corrosion, wear, and oxidation. Performance and properties of the coatings depend upon their composition, microstructure, and deposition condition. Simultaneous co-evaporation of multiple ingots of different compositions in the high energy EB-PVD chamber has brought considerable interest in the architecture of functional graded coatings, nano-laminated coatings, and design of new structural materials that could not be produced economically by conventional methods. In addition, high evaporation and condensate rates allowed fabricating precision net-shaped components with nanograined microstructure for various applications. Using EB-PVD, nano-grained rhenium (Re) coatings and net-shaped components with tailored microstructure and properties were fabricated in the form of tubes, plates, and Re-coated spherical graphite cores. This paper will also present the results of various metallic and ceramic coatings including chromium, titanium carbide (TiC), titanium diboride (TiB2), hafnium nitride (HfN), titanium-boron-carbonitride (TiBCN), and partially yttria stabilized zirconia (YSZ) TBC coatings deposited by EB-PVD for various applications.

Thermal Conductivity and Thermal Stability of Zirconia and Hafnia Based Thermal Barrier Coatings by EB-PVD for High Temperature Applications

Zirconia and hafnia based thermal barrier coating materials were produced by industrial prototype electron beam-physical vapor deposition (EB-PVD). Columnar microstructure of the thermal barrier coatings were modified with controlled microporosity and diffuse sub-interfaces resulting in lower thermal conductivity (20-30% depending up on microporosity volume fraction), higher thermal reflectance (15-20%) and more strain tolerance as compared with standard thermal barrier coatings (TBC). The novel processed coating systems were examined by various techniques including scanning electron microscopy (SEM), X-ray diffraction, and thermal conductivity by laser technique, hemispherical reflectance and thermal cyclic tests. The test results showed the tailored-microstructural TBC offered superior performance over the conventional thermal barrier coatings (ZrO2 -8 wt.% Y2O3).

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...