Simulation of Micro-Precoating Effect on Temperature Distribution in Plasma Thermal Spraying

: The effect of micro-precoating on temperature distribution of the plasma thermal spraying was simulated. After the plasma thermal spraying test with SS304 as precoating, the effect of micro-precoating on the wear loss and fatigue had been studied preliminarily. Results show that the surface temperature with micro-precoating is higher than that of without micro-precoating. Meanwhile, the heat affected zone with precoating is wider which effectively restrains the microcracks forming in the succedent coating. Subsequently, the wear and fatigue of the plasma thermal spray coating are significantly increased with the optimum micro-precoating thickness.


INTRODUCTION *
In recent years, micro technology has become one of the key disciplines, having a significant effect on the development of new products and production technologies, as well as on the introduction of new methods for medical treatment and diagnostics. Moreover, due to the enormous potential for new applications, in future, micro technology will increasingly affect our daily life with an impact, comparable to that of the industrial revolution in the 20 th century, or the development of information technologies in the 21 st century [1][2][3][4].
To date, a key issue associated with miniaturization is the tremendous increase in the heat dissipation per unit volume. Microstructures may enable engineered materials with unique thermal properties to allow significant enhancement or reduction of the heat flow rate [5][6][7][8][9][10]. Therefore, knowledge of thermal transport from the micrometer scale and thermal properties of microstructures is of critical importance to future technological growth. Also, the effect of micro-parts on the macrostructure, especially for the micro-precoating in the thermal spraying, has not been investigated in detail yet. Consequently, the complicated problems due to microprecoating should be overcome. This research aims at investigating the effect of micro-precoating on the temperature distribution in the thermal spraying to overcome

EXPERIMENTAL MATERIAL AND PROCEDURES
The substrate was AISI 1045, and stainless steel 304 was used as precoating with 50 µm in thickness. The substrate was machined into 80 mm×60 mm×7.95~8 mm and carefully cleaned by acetone and pure ethyl alcohol to remove any contaminants on its surface. EIS-200ER plasma thermal spraying system made by Elionix, with accelerated voltage 2500 V, Ar gas flow rate 0.61 SCCM, current 16~20 mA and focus spot diameter of approximately 8 mm, was used to precoat the micro-stainless steel 304 film and irradiate the AISI 1045 steel. After irradiated, the surface morphology of precoating was observed by Scanning Electron Microscope (SEM) JEOL/JSM-5600, and the temperature was measured by apparatus attached with EIS-200ER.

Equations for Temperature Distribution
Using energy balance, a differential equation can be obtained for the steady temperature distribution in a homogeneous isotopic medium, that is Where the boundary conditions are ! s 1 After Eq. 2 is discrete for the element, and according to where S: isothermal boundary, B: the heat-input, c: the conductive and r: the irradiative.

Classical Size Effect on Thermal Conductivity Based on Geometric Considerations
In the ballistic transport limit when δ<ξb as shown in Figure 1, if it is assumed that the mean free path in the film is the same as the micro-precoating thickness δ, i.e., ! f = " , then the conductivity ratio can be obtained as Knudsen number, adopted from the theory of rarefied gas dynamics, for electrons or phonons. In the intermediate region, the Matthiessen's rule can be applied 1 However, the distribution of free path is not taken into consideration for simplicity in the above equations. In other words, all paths are assumed to be the same as the mean free path in the bulk. When δ>ξb, it may be assumed that all energy carriers originate from the boundary. From Figure 1, it is obtained ! " The free paths should be averaged over the hemisphere, and the weighted average can be written and evaluated as follows: Applying Matthiessen's rule, it is obtained This equation, however, cannot be applied for small values of Kn since ln(Kn) becomes negative. Assumed Eq. is applicable for K n > 5 . When K n < 1 , it may be used Where m ! 3 for thin films. Eq. 6 can be considered as a special case of Eq. 10 with m = 1 . In this research, the direction of transport and the anisotropic feature due to size effect are not considered due to spraying size.
The results calculated are plotted in Figure 2 to illustrate the size dependence of the effective thermal conductivity.

Hypothesis and Mesh
Based on the situations during the plasma thermal spraying and mainly focused on the effect of precoating, it is supposed that the spraying particles are considered as a Gaussian heat source with 8 mm in diameter to simplify the effect of high temperature spray particles on the micro-precoating. Meanwhile, the sample size is 80 mm (x) × 60 mm (y) × 7.95~8 mm (z), its finite element (FE) mesh is shown in Figure 3.

Temperature Distribution
The simulation results are shown in Figure 4. It shows that the temperature with the precoating is higher than that of without precoating. Meanwhile, the heat affected zone increases correspondingly which can be observed more obviously in Figures 4e, f. As a result, the heat input into the substrate will increase in the wider area to effectively restrain the cracks forming in the succedent plasma thermal spraying. The measured temperature results are shown in Figure 5. It shows that the measured results match well with the simulated results.
Moreover, with the heat effect of the plasma spraying, the micro-precoating will be melted firstly, then under the effect of surface tension in nanoscale, it will be separated and formed numerous nanoparitcles as shown in Figure 6. Therefore, the conductivity related to Eq. 6 and Eq. 10 should be revised. Consequently, the effect of micro-precoating will become more complicated which is neglected in this paper. In addition, if the precoating thickness is too thin, it will not have any distinct effect on the succedent plasma thermal spraying. When the precoating is too thick, the succedent coating will be cracked and peeled off resulted in failure coating. Therefore, the effect of precoating thickness on the temperature distribution is not listed here.

Wear Test
Wear was studied using a Cameron-Plint TE67 wear test rig (Made in UK) under dry conditions with a sliding speed 1.34 Ultrasonic cleaner with 28-34 Hz frequency was used for cleaning samples. All well coated samples are ultrasonically cleaned in an acetone bath for 2 min, and carefully dried before/after wear testing. The weight of samples is measured by the Sartorius electronic weight before and after the wear testing with a minimum accuracy of 0.0001 g. The results are listed in Table 1 and plotted in Figure 7. The corresponding precoating parameters are expressed in Table 2. Generally speaking, the wear loss with precoating is lower than that without precoating. However, when the thickness of precoating exceeds 50 µm, the wear loss will be increased further, especially for the thicker thickness as shown in Figure  7 line E resulted in the lower quality coating.

Fatigue Test
The fatigue test was also carried on the Cameron-Plint TE67 wear test rig, where the maximum heating temperature is 800 °C with 2 min preservation, and the heating speed is 20 °C/min. Subsequently, samples are put into the 25 °C water. After cooling, samples are cleaned by acetone and alcohol, and dried by the drier. Finally, samples are observed by optical microscopy. The results are listed in Table 3. It shows that with the precoating the fatigue properties of spray coating are increased to some extent, especially for the   coating with 50 µm precoating. But, for spray coating with 100 µm precoating, the quality is decreased similar to that of wear test listed in Table 1.

CONCLUSION
Simulation of the micro-precoating effect on the temperature distribution during the plasma thermal spraying was investigated. It shows that the surface temperature with micro-precoating is higher than that of without microprecoating. Meanwhile, the heat affected zone with precoating is wider resulted in restraining the microcracks forming effectively in the succedent thermal spraying. Furthermore, the wear and fatigue of thermal spray coating are significantly increased with the optimum micro-precoating thickness.