Platinum coatings are widely used on alumina (Al2O3) sheaths for thermocouples in the U.S. glass industry to protect the thermocouple wires and alumina sheath from corrosion and dissolution of the temperature-sensing unit. The cost associated with platinum coatings can be prohibitively high considering the steps necessary at glass plants to maintain and secure an inventory of platinum. There also is a need to improve the performance of the platinum-coated Al2O3 sheaths, because the failure rate of the thermocouples can be as high as 50%. These issues are driving the glass industry to search for alternative materials that can replace platinum and still provide the durability and performance needed to survive in an extremely corrosive glass environment.
Investigations by Y.S. Park et al. demonstrated that molybdenum disilicide (MoSi2) has performance properties in molten glass similar to that of some refractory materials currently used in glass-processing applications . Molybdenum disilicide is a candidate high-temperature material for use in such applications because it has a high melting temperature (2030C, or 3686F), relative low density (6.24 g/cm3, or 0.225 lb/in.3), high thermal conductivity (52 W/m?K), a brittle-to-ductile transition near 1000C (1830F) and it is stable in a variety of corrosive and oxidative environments [2,3]. Additionally, MoSi2 is significantly less expensive than a platinum coating.
Plasma spraying is a very effective method to produce MoSi2 and MoSi2 composite coatings and spray-formed components . For example, studies of plasma spray-formed MoSi2-Al2O3 composite gas-injection tubes show they have enhanced high temperature thermal shock resistance when immersed in molten copper and aluminum . The composite tubes outperform high-grade graphite and silicon-carbide (SiC) tubes when immersed in molten copper and had similar performance to high-density graphite and mullite when immersed in molten aluminum. Energy-absorbing mechanisms such as debonding (between the MoSi2 and Al2O3 layers) and microcracking in the Al2O3 layer contribute to the ability of the composite to absorb thermal stresses and strain energy during the performance test (Fig. 1). Molybdenum disilicide and alumina are chemically compatible and have similar thermal expansion coefficients [6,7].
A MoSi2 coating on the Al2O3 protective thermocouple sheath is required to have optimal performance in both a high-temperature (>1300C, or 2370F) oxidizing environment (above the glass line) and a highly corrosive molten glass environment (below the glass line) as the thermocouple is immersed in the molten glass. Therefore, a graded coating of Al2O3 to MoSi2 was evaluated in research at Los Alamos National Laboratory to enhance coating performance in molten glass. The graded coating microstructure reduces residual stresses that can develop during the spray deposition process-stresses that can cause cracking and spalling of the coating, limiting coating service life.
Coating production and testing
Conventional plasma spraying allows the flexibility of producing a variety of MoSi2-Al2O3 microstructures including laminate and graded structures. Graded structures were produced using a plasma torch (Praxair Surface Technologies SG100) mounted on a six-axes robot. Processing gases and powder dispensing rate from two powder hoppers was monitored and controlled by means of a computer control system. Pure Al2O3 is first deposited at a powder hopper-rotation speed of 0.8 rpm. After depositing Al2O3 for an initial period, MoSi2 is gradually introduced into the plasma torch by increasing the powder hopper-rotational speed for MoSi2 and decreasing the rotational speed for Al2O3. The powder hopper speed for MoSi2 subsequently reaches 0.8 rpm and the Al2O3 powder dispensing speed goes to zero. Pure MoSi2 is then deposited on the outside diameter of the tube. Argon was used both for the plasma generating gas (40 standard liters per minute, or slm) and as a carrier gas for the MoSi2 and Al2O3 powders (1 to 4 slm). Powders were deposited on a 12.7 mm OD _ 9.5 mm ID (0.5 _ 0.375 in.) graphite substrate, while monitoring the substrate temperature using an infrared camera.
Mechanical properties of graded MoSi2-Al2O3 structures were determined from tests on 25.93 mm OD _ 12.8 mm ID _ 10.76 mm wide (1 _ 0.5 _ 0.4 in.) C-rings machined from the sprayed tube samples. A 1.64 critical b/(ro-ri) ratio was within the required range of 1 to 4. C-ring samples were machined and tested in diametrical compression in accordance with ASTM Standard C 1323-96 using a hydraulic test frame (Instron) at a cross-head speed of 0.125 mm/min [strain rate ~ 0.316 x 10(-4) s(-1)]. Machine compliance was corrected using a standard Al2O3 sample of known stiffness.
Twelve samples for each composite tube and four samples of monolithic plasma-sprayed MoSi2 and Al2O3 (for comparison) were tested. A standardized student-t test using a normal, or gaussian, frequency distribution often is used in statistical analysis, but there is no theoretical or experimental justification to use this approach in problems involving fracture . The use of a normal distribution often is inappropriate in analyzing fracture problems associated with plasma-sprayed ceramic materials because of the presence of multiple flaw populations. Therefore, strength distributions in the coated and uncoated samples were obtained using a Weibull statistical approach . The Weibull distribution is more appropriate (and conservative) in this case because it does not require a normal distribution of the flaw population.
Figure 2 shows macrographs of two types of MoSi2- Al2O3 graded composites produced using plasma spraying. The white phase is the Al2O3 and the dark phase is MoSi2. Figure 2a shows a sample cross section of a layered, graded microstructure consisting of discrete individual layers graded from Al2O3 to MoSi2. Figure 2b shows a cross section of a continuously graded structure where pure Al2O3 was first deposited on the graphite rod followed by increasing amounts of MoSi2, until pure MoSi2 is deposited on the outside diameter of the tube. Figure 3 shows a typical microstructure produced in the graded region between the MoSi2 and the Al2O3. A layered type of microstructure is produced when individual molten particles are flattened as they impact the substrate or previously deposited material.
Three sources of residual stresses in plasma sprayed material are: quenching stresses that occur when individual particles are rapidly cooled upon impacting the substrate surface; differential thermal contraction between the deposited material and substrate; and volume changes associated with any solid state phase transformation . Residual stresses can lead to deformation of the coated workpiece and can result in spalling and cracking of the coating. In addition, various types of coating performance indicators, such as adhesion strength, resistance to thermal shock under thermal cycling and erosion resistance, are strongly influenced by the nature of the residual stresses. At elevated temperatures (> 1000C, or 1830F), MoSi2 goes through a brittle-to-ductile transformation, and can accommodate some of the residual stresses that occur during the deposition process and during high-temperature use of the material. Combining MoSi2 with Al2O3 can enhance the high temperature performance of Al2O3 protective thermocouple sheaths by improving the poor thermal shock behavior of Al2O3.
Thermal and mechanical performance
Layered and continuous FGM strength distribution plots are shown in Figure 4. Both continuous and layered FGM microstructures have similar mean Weibull strengths (_f~70 MPa, or 10 ksi), which were calculated using the following equation: _f = a-1/_ _[1 + 1/_] where beta (_) is the Weibull modulus and gamma (_) is a function of the sample size . However, the spread of the data for the continuously graded material was smaller, thus, a larger Weibull slope (13.38 for the continuously graded samples vs. 7.635 for the layered graded samples). FGM fracture energy (qualitatively determined from the area under the load-displacement plot of the C-ring tests) is significantly higher (~3 times) than that of monolithic Al2O3 and MoSi2.
Fracture energies of the monolithic and graded coatings are listed in Table 1. Independent fracture toughness tests are being conducted to validate these results and to determine if the graded microstructures have R-curve behavior.
Continuously graded and layered FGM fracture surfaces are shown in Figure 5. The fracture surface contained extensive microcracking and roughening in the center portion of the C-ring. Increased toughening of the composite is believed to be a direct result of microcracking. This work indicates that the strength and toughness of FGM tubes meet the performance requirements of this application.
SIDEBAR: Understanding the glass-corrosion process
Early studies conducted on MoSi2 in a molten glass environment indicated that oxidation of MoSi2 below and above the glass line is acceptable, but the corrosion rate at the glass line is unacceptably high. The high glass-line oxidation provided challenges in the development of thermocouple sheaths. The answers to some of these challenges lie in understanding the mechanisms involved in the glass-corrosion process. A schematic of a phenomenological model describing the oxidation mechanisms occurring on MoSi2 above, below and at the glass line (in a borosilicate glass melt environment) is shown in figure. Above the glass line (air environment), a SiO2 coating protects the MoSi2 from continuous oxidation. At a temperature around 500C (930F), the protective SiO2 layer is disrupted and pesting of the MoSi2 can occur. Below the glass line, molybdenum metal, oxides and silicides form in a silicon-depleted layer. The silicon-depleted layer consists of a mixture of phases (Mo5Si3, Mo3Si and Mo), which are stable due to low oxygen activity .
The high flux of molten glass at the glass line increases the oxygen activity, resulting in increased oxidation rates. The oxidation rate is further accelerated by the dissolution of the protective SiO2 layer as a result of the convective flow of glass at the glass line.
Further work is aimed at producing a protective MoSi2 coating to eliminate the high corrosion rate at the glass line. These activities include developing composite MoSi2 coatings and higher viscosity glasses to minimize glass line corrosion rates.