André G. McDonald

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.

A Review of Thermal Spray Metallization of Polymer-Based Structures

A literature review on the thermal spray deposition of metals onto polymer-based structures is presented. The deposition of metals onto polymer-based structures has been developed to enhance the thermal and electrical properties of the resulting metal-polymer material system. First, the description of the thermal spray metallization processes and technologies for polymer-based materials are outlined. Then, polymer surface preparation methods and the deposition of metal bond-coats are explored. Moreover, the thermal spray process parameters that affect the properties of metal deposits on polymers are described, followed by studies on the temperature distribution within the polymer substrate during the thermal spray process. The objective of this review is devoted to testing and potential applications of thermal-sprayed metal coatings deposited onto polymer-based substrates. This review aims to summarize the state-of-the-art contributions to research on the thermal spray metallization of polymer-based materials, which has gained recent attention for potential and novel applications.

Characterizing the performance of a single-layer fabric system through a heat and mass transfer model - Part II: Thermal and evaporative resistances

In Part I of this work, a heat and mass transfer model was developed to calculate the thermal and evaporative resistances of a single-layer fabric system. Using this model, the effects of environmental conditions, air gap, and material properties on the thermal and evaporative resistances have now been studied. The thickness of the air gap and that of the fabric layer were shown to contribute significantly to both the thermal resistance and evaporative resistance. The occurrence of natural convection in the air gap can cause decreases in thermal and evaporative resistances, and needs to be considered to determine the optimal air gap thickness. The porosity of the fabric layer has a distinct effect on the two resistances, and is an excellent property to help achieve both thermal protection and comfort. This work provides the fundamental basis for the optimization of garment fit and material properties to achieve good performance of the clothing system.

Characterizing the performance of a single-layer fabric system through a heat and mass transfer model - Part I: Heat and mass transfer model

A mathematical model was developed to study the coupled heat and moisture transfer through a fabric system that consists of a single layer of fabric and an air gap. Properties of air and moisture are sensitive to temperature and, hence, were assumed to be functions of local temperature. Therefore, the model is applicable to a broad range of boundary conditions. A numerical scheme was proposed to solve the distributions of temperature and water vapor concentration throughout the layers, from which the thermal and evaporative resistances of the fabric system were evaluated. Experiments were conducted for two particular fabrics using a sweating guarded hotplate, and the data show good agreement with the model predictions, suggesting that the heat and mass transfer model is capable of accurately predicting thermal and evaporative resistances for the single-layer fabric system.

Structure, phases, and mechanical response of Ti-alloy bioactive glass composite coatings.

Porous titanium alloy-bioactive glass composite coatings were manufactured via the flame spray deposition process. The porous coatings, targeted for orthodontic and bone-fixation applications, were made from bioactive glass (45S5) powder blended with either commercially pure titanium (Cp-Ti) or Ti-6Al-4V alloy powder. Two sets of spray conditions, two metallic particle size distributions, and two glass particle size distributions were used for this study. Negative control coatings consisting of pure Ti-6Al-4V alloy or Cp-Ti were sprayed under both conditions. The as-sprayed coatings were characterized through quantitative optical cross-sectional metallography, X-ray diffraction (XRD), and ASTM Standard C633 tensile adhesion testing. Determination of the porosity and glassy phase distribution was achieved by using image analysis in accordance with ASTM Standard E2109. Theoretical thermodynamic and heat transfer modeling was conducted to explain experimental observations. Thermodynamic modeling was performed to estimate the flame temperature and chemical environment for each spray condition and a lumped capacitance heat transfer model was developed to estimate the temperatures attained by each particle. These models were used to establish trends among the choice of alloy, spray condition, and particle size distribution. The deposition parameters, alloy composition, and alteration of the feedstock powder size distribution had a significant effect on the coating microstructure, porosity, phases present, mechanical response, and theoretical particle temperatures that were attained. The most promising coatings were the Ti-6Al-4V-based composite coatings, which had bond strength of 20±2MPa (n=5) and received reinforcement and strengthening from the inclusion of a glassy phase. It was shown that the use of the Ti-6Al-4V-bioactive glass composite coatings may be a superior choice due to the possible osteoproductivity from the bioactive glass, the potential ability to support tissue ingrowth and vascular tissue, and the comparable strength to similar coatings.

Gas-Substrate Heat Exchange During Cold-Gas Dynamic Spraying

In this study, the temperature distribution of the surfaces of several substrates under an impinging gas jet from a cold spray nozzle was determined. A low-pressure cold-gas dynamic spraying unit was used to generate a jet of hot compressed nitrogen that impinged upon flat substrates. Computer codes based on a finite differences method were used to solve a simplified 2D temperature distribution equation for the substrate to produce nondimensional relationships between the surface temperature and the radius of the impinging fluid jet, the axial velocity of the cold spray nozzle, the substrate thickness, and the heating time. It was found that a single profile of the transient nondimensional maximum surface temperature could be used to estimate the dimensional maximum surface temperature, regardless of the value of the compressed gas temperature. It was found further that, as the thermal conductance of the substrate increased, the maximum surface temperature of the substrate beneath the gas jet decreased. Heat exchange between the substrate and the compressed gas jet during motion of the nozzle to produce heat conduction within the substrate was characterized by the nondimensional Peclét number. It was found that lower Peclét numbers produced higher temperatures within the substrate. The close agreement of the numerical results with the experimental results suggests that the nondimensionalized results may be applied to a wide range of conditions and materials.

Deposition of Electrically Conductive Coatings on Castable Polyurethane Elastomers by the Flame Spraying Process

Deposition of metallic coatings on elastomeric polymers is a challenging task due to the heat sensitivity and soft nature of these materials and the high temperatures in thermal spraying processes. In this study, a flame spraying process was employed to deposit conductive coatings of aluminum-12silicon on polyurethane elastomers. The effect of process parameters, i.e., stand-off distance and air added to the flame spray torch, on temperature distribution and corresponding effects on coating characteristics, including electrical resistivity, were investigated. An analytical model based on a Green’s function approach was employed to determine the temperature distribution within the substrate. It was found that the coating porosity and electrical resistance decreased by increasing the pressure of the air injected into the flame spray torch during deposition. The latter also allowed for a reduction of the stand-off distance of the flame spray torch. Dynamic mechanical analysis was performed to investigate the effect of the increase in temperature within the substrate on its dynamic mechanical properties. It was found that the spraying process did not significantly change the storage modulus of the polyurethane substrate material.

Mathematical model and sensor development for measuring energy transfer from wildland fires

Current practices for measuring high heat flux in scenarios such as wildland forest fires use expensive, thermopile-based sensors, coupled with mathematical models based on a semi-infinite-length scale. Although these sensors are acceptable for experimental testing in laboratories, high error rates or the need for water cooling limits their applications in field experiments. Therefore, a one-dimensional, finite-length scale, transient-heat conduction model was developed and combined with an inexpensive, thermocouple-based rectangular sensor, to create a rapidly deployable, non-cooled sensor for testing in field environments. The proposed model was developed using concepts from heat conduction and with transient temperature boundary conditions, to avoid complicated radiation and convection conditions. Constant heat flux and tree-burning tests were respectively conducted using a mass loss cone calorimeter and a propane-fired radiant panel to validate the proposed analytical model and sensor as well as test the sensor in a simulated forest fire setting. The sensor was mounted directly beside a commercial Schmidt–Boelter gauge to provide data for comparison. The proposed heat flux measurement method provided results similar to those obtained from the commercial heat flux gauge to within one standard deviation. This suggests that the use of a finite-length scale model, coupled with an inexpensive thermocouple-based sensor, is effective in estimating the intense heat loads from wildland fires.

Performance testing of wildland fire chemicals using a custom-built heat flux sensor:

A simple and effective laboratory test methodology was developed for differentiating wildland fire chemicals based on the ignition time of vegetative fuel samples. The test apparatus consisted of an electric-powered radiant heater that was used to produce a uniform radiant thermal load to ignite the vegetative fuel samples. The samples, treated with wildland fire chemicals, were mounted on to a load cell to determine the transient mass loss during the combustion process. A custom-built heat flux sensor, that was modified and tested to reduce high errors, was used to determine the time to flaming ignition. The time to flaming ignition was also measured using transient mass loss data of the vegetative fuel samples. Statistical t-test analysis was conducted on the time to flaming ignition to determine whether the results were statistically significant for the different chemical treatments. The results indicated that the test methodology allowed for effective differentiation between the wildland fire chemical treatments by comparing their mean ignition times. The narrow standard deviations of the average ignition times suggested that the test methodology was able to produce repeatable results.

Heat Transfer Coefficient of an Under-Expanding Cold Spray Air Jet on a Flat Substrate

A two-dimensional heat conduction model was developed to determine the transient temperature distribution within a flat substrate that was exposed to the impingement of a cold spray hot air jet during the deposition process. The credibility of employing average heat transfer coefficient in mathematical modelling and prediction of the surface temperature profile of a substrate was studied. Moreover, the condition under which the effect of the presence of the in-flight particles on the heat transfer coefficient of the underexpanding air jet can be neglected was investigated. In this regard, a cold spray unit was used to generate a supersonic air jet. A twodimensional heat conduction model was developed and solved by using Green’s functions to determine the temperature distribution within the substrate. By applying a surface integral to the analytically-estimated spatially-varying heat transfer coefficient of an underexpanding cold spray air jet, the average heat transfer coefficient was determined. Both the average and spatially-varying heat transfer coefficients were used separately in the model to predict the transient surface temperature profile of the substrate. It was shown that when the Stokes number of the particles is sufficiently small, the effect of the presence of the particles on the heat transfer coefficient of the impinging dilute air-particle jet can be neglected. It was further found that the surface temperature that was predicted by using the average heat transfer coefficient, unlike the spatially-varying heat transfer coefficient, produced large errors, especially at higher distances from the stagnation point of the air jet on the substrate surface.