Thermal Processing - Capabilities
Flow Field Diagnostics -Diagnostics are extremely important for accurately verifying and benchmarking CFD calculations or particle behavior and understanding complex phenomena such as particle heating and in-flight oxidation. Critical variables such as temperature, velocity, entrained air fraction and concentrations of very reactive species like atomic oxygen are difficult to measure because of brief particle residence time (on the order of 1 ms), rapid heating, and the steep gradients in HVOF and plasma flow fields.
INL researchers use flow field visualization techniques such as pulsed laser schliren to obtain qualitative information on jet behavior and turbulence and quantitative information on jet spreading and laminar-to-turbulent transition. Enthalpy probes integrated with a mass spectrometer are used to obtain information on total entrained air fraction, and with the assumption of local thermodynamic equilibrium yield temperature and velocity information. High-resolution coherent Thomson laser scattering provides a direct measurement of gas temperature and very high-resolution measurement of the Doppler shift due to bulk gas movement provides gas velocity. Nonlinear spectroscopic techniques including Coherent-Anti-Stokes-Raman Spectroscopy (CARS) provide concentration and temperature of molecular species like oxygen and nitrogen. Atomic species concentrations are obtained by multiple-photon laser induced fluorescence (LIF).
Particle Diagnostics -Over a number of years, the INL has developed several particle diagnostic techniques applicable for scientific studies as well as routine coating fabrication. The most sophisticated of these techniques uses a laser based Phase Doppler Particle Analyzer (PDPA) to acquire simultaneous information on particle size and velocity. INL researchers developed sophisticated timing and logic circuitry to enhance the PDPA capabilities and integrated a high-speed two-color pyrometer system into the PDPA system design. A schematic of the instrument appears in Figure 1. Using this modified PDPA system, researchers can obtain temperature measurements and ensure that size, velocity and temperature data all originate with the same particle. This diagnostic technique is advantageous because particle size can be measured directly and independently of particle optical characteristics such as emissivity. However the assumption of spherical particles is implicit in the measurement. The diagnostic technique can also identify the velocity and diameter of "cold" particles (particles that are not hot enough to provide temperature information) through laser interrogation. In addition, size and velocity information can be obtained on particles that are entrained directly in the highly luminous hot regions of a plasma jet.
Closed Loop Process Control -Process control is an ongoing research and development effort at the INL. Plasma-spray processing is inherently complex because plasma-particle interactions can cause variations significant enough to limit process repeatability. Such variations occur in the particle-state from run-to-run and during long continuous depositions for large parts. The limited ability to maintain a narrow operating window precludes applications that require tight control of coating properties, and increases the complexity in developing new process recipes to achieve a specific set of coating performance objectives. INL’s strategy is to improve plasma spray processing controls through real time diagnostics. Such capability should improve yield, enable engineering capabilities to develop new coating structures, and reduced time and cost to market. We have developed diagnostics specifically for this purpose and have now implemented two generations of closed loop process controllers based on these measurements. We can directly monitor and control the process degrees of freedom such as particle velocity, temperature, and shape and trajectory of the spray pattern, that determine coating properties.
LAVA CFD Code - The ability to separate variables and examine the influence of each variable separately in a controlled manner is the primary advantage offered of computational treatments of highly complex phenomena such the plasma spray process. Computational tools can provide insight into phenomena that are either difficult to measure or that cannot be directly measured. Particle melt fraction shown in Figure 2, or the location of the melt or solidification fronts or the internal distributions of temperature in a particle are examples. The Lava CFD code was developed for this purpose. LAVA code is a greatly enhanced version of an earlier spray combustion model. LAVA solves the complete, time-dependent, compressible Navier-Stokes equations for a multi-component chemically reacting ideal gas with temperature dependent properties. The code is multi-dimensional including 2D, axisymmetric with swirl, and fully 3D geometry. Turbulent mixing is modeled using either a k-e or subscale grid turbulence model. A self-consistent ambipolar ion-electron diffusion model is included and the heavy particle temperature is allowed to differ from the electron temperature. The solution technique uses explicit finite difference numerics and a unique computationally efficient algorithm that handles the disparate time scales of mixed "fast" and "slow" chemistry has been implemented.
The particle model allows for multiple simultaneous injection locations and particle types. The stochastic injection model allows the user to specify the distribution of particle size and density, injection velocity, and injection direction in three space dimensions. Particles are modeled as discrete Lagrangian entities, which exchange momentum and energy with the plasma. A statistical approach is used to approximate particle dispersion by turbulent gas motion. The concept of a "computational particle" is employed, where each computational particle is representative of a group of similar real particles. Sampling from the specified distributions of particle properties stochastically generates computational particles. The particle behavior model now includes the calculation of temperature distribution and the location of the melt or solidification front in each particle. A mass transfer model accounts for both the effect of evaporation on plasma-particle heat transfer and allows for realistic estimation of mass loss at temperatures less than the particle boiling point. Heterogeneous surface chemical reactions, such as oxidation, and diffusion of oxygen or other chemical species into the particle have also been included.
Coating Characterization -Recent INL research efforts into physical characterization are focused on quantifying the mechanical properties (Young’s modulus, etc.) and residual stress state of coatings. The total stress state in a coating/substrate is comprised of the quench stress and the coefficient of thermal expansion (CTE) mismatch stress. The residual stress-state of a coating is experimentally determined by a laser-based coupon curvature measurement technique, Figure 3. The measurement technique is integrated with a vacuum furnace making it possible to evenly heat the substrate/coating unit without the effects of oxidation. Quench stress and thermal mismatch stress can be estimated by measuring the curvature at various temperatures between room temperature and the temperature of the substrate during deposition. Measuring the curvature as a function of temperature also permits direct calculation of the coating’s temperature CTE rather than relying on tabulated values for bulk material. This is particularly important because thermally sprayed material properties can vary greatly from those of a bulk material. Complementary work is addressing the experimental determination of the modulus and thermal transport properties of thermally sprayed materials. INL also employs the traditional suite of physical characterization methods involving optical or SEM examination of microstructure coating samples, measuring micro hardness, adhesion, and quantifying phase content and chemical composition using x-ray diffraction and XPS, Auger, etc.