Plasma spray for depositing thermal barrier coatings features large distributions of particle states that result in significant variations in coating quality. These variations arise from distributions of particle sizes, large spatial gradients of plasma thermal-fluid fields, and temporal variations of the arc and jet. This paper describes a simplified approach for studying how particle state distributions are influenced by torch conditions and powder distributions, and the implications for deposition rate monitoring and control. The approach combines a simplified jet model with a more detailed particle model. The important fluid-thermal spatial gradients in the plasma jet are captured using a three zone model: a core region, modeled by growth of a turbulent shear layer around a laminar core, a transition region and a similarity region. Plasma-particle momentum and thermal interactions, particle phase transitions, internal particle temperature gradients, and collapse of in-flight hollow particles have been modeled using a multi-lumped particle model. Effects of distributions of particle size, particle morphology, injection velocity, and carrier gas flow were studied for YSZ spray in an Ar-He plasma. The results provide guidance on sensor design and operation and on approaches for plume location control.

This paper investigates the need and requirements for developing an improved sensing scheme for plasma spray, which would aid in compensating for the large variations in deposition rate. Experimental evaluation of a variety of current sensors reveals poor correlation with deposition rate. Important issues that explain these results include reconsidering what particle state characteristics should be measured and determining what sensing characteristics are required to achieve better correlation with deposited mass. A new particle flux sensor is proposed that meets these challenges is evaluated.

Real-time control offers the potential to reduce plasma spray variations that affect yield and coating quality. Important factors for designing such controllers are discussed including dominant nonlinearities, cross-coupling interactions, and sensor issues. The performance of several alternative strategies to achieve better coating thickness control are evaluated.

The plasma-spray process features complex plasma-particle interactions that can result in process variations that limit process repeatability and coating performance. This paper reports our work on the development of real-time diagnostics and control for the plasma spray process. The strategy is to directly monitor and control those degrees of freedom of the process that are observable, controllable, and affect resulting coating properties. This includes monitoring of particle velocity and temperature as well as the shape and trajectory of the spray pattern. Diagnostics that have been developed specifically for this purpose and integrated with a closed loop process controller are described.

This paper investigates the advantages that an advanced control system can provide for plasma spray deposition in terms of variation reduction and greater ability to engineer coating structure. We report our work on implementing a feedback control system that automatically adjusts system inputs to maintain the desired particle states in spite of process variations. The limitations and performance capabilities of both feed forward and real-time control are evaluated. Important system characteristics needed to develop such controllers are discussed including dominant nonlinearalities, dynamics, cross-coupling, distributions, and sensor issues. Performance is evaluated in terms of engineering coating structure and production objectives.

An advanced closed loop control system that enables tight control of the particle and substrate states has been developed. This unique capability allows deposition of coatings under very controlled conditions. This enables the construction of detailed process/property maps that can lead to a fundamental understanding of the formation mechanisms of key microstructural features during the plasma deposition process. The microstructural development during processing is discussed in light of the physics of microcrack formation during plasma deposition, including the effect of particle and substrate states on splat solidification, thermal gradients and residual stresses.