Material Recession in a High-temperature, High-velocity Water ...

Material Recession in a High-temperature, High-velocity Water Vapor EnvironmentA Technical ReportIn STS 4600Presented toThe Faculty of theSchool of Engineering and Applied ScienceUniversity of VirginiaIn Partial Fulfillment of the Requirements for the DegreeBachelor of Science in Engineering ScienceJack ValentineApril 11 th , 2012On my honor as a University student, I have neither given nor received unauthorized aid on thisassignment as defined by the Honor Guidelines for Thesis-Related AssignmentsSigned: __________________________________________________Date: _____________Approved: ________________________________________________Elizabeth Opila, Materials Science and EngineeringDate: _____________

2AbstractA laboratory testing device was created to simulate the environment of a combustionturbine engine. Liquid water was pumped into a furnace, moved through a small diametercapillary, vaporized and expelled as a gas at accelerated velocities. A SiO 2 sample was tested inthe device for 170 hours at 1300°C and produced visible surface degradation due to avolatilization reaction which allowed estimations for the recession and weight loss rates. Therates were comparable to theoretical predictions. These results demonstrate the device’s capacityfor future testing of potential engine materials in a low-cost laboratory setting.IntroductionFor combustion turbine engines, increased efficiency requires higher operatingtemperatures. 1 Having reached the upper limits of current high-temperature metals, new ceramicmaterials, which offer the possibility of higher engine operation temperature, are required forfurther improvement in engine efficiencies. These materials still need significant characterizationbefore they can be implemented effectively into designs. 2 The key problem with using ceramicsis they react with water vapor (a product of combustion reactions) and volatilize rapidly. 3 Thispaper outlines a method for creating a laboratory testing device that replicates a combustionenvironment.Silicon carbide (SiC) and similar ceramic composites are leading candidates for engineapplication due to their high temperature capability. However, SiC is prone to rapid deteriorationthrough the following reactions with water vapor. 4( ) ( ) ( ) (1)( ) ( ) (2)

3As shown in Eqn. 1, the SiC reacts with water vapor to form an oxide layer (SiO 2 ). This oxidelayer then reacts with water vapor as shown in Eqn. 2 and volatilizes into Si(OH) 4 . The high rateof these reactions can lead to severe material loss and has been well documented in the papers ofOpila 7 and Robinson and Smialek. 8This paper focuses on a method for observing high-temperature materials’ reaction withwater vapor in an environment typical of combustion turbines. Based on the design of dos Santoset al. 2 , Ferber and Lin 5 and Sudhir and Raj 6 the objective is to create observable rapid recessionand compare the results with theoretical predictions.To observe these degradation mechanisms, there are several design objectives. Theoxidation mechanisms described in Eqn. 1 and 2 have a strong dependence on a variety of factorsincluding: temperature, water vapor partial pressure, and gas flow velocity. 7 It is crucial toreplicate these factors of a combustion environment in the testing device to create meaningfulresults. This device will additionally act as a faster alternative to very expensive burner rig testsand allow for more precise control of independent variables. 2 It will also allow for observation ofthe reactions in a controlled environment.Experimental ProcedureThe testing device design shown below in Fig. 1 uses a horizontal tube furnace (CMFurnaces Inc., Bloomfield, NJ, Model #100803) with a 1m long alumina tube.Figure 1: Depiction of Testing Device

̇4The stainless steel end cap has two inlet openings; the top one holds a fused quartz (SiO 2 )capillary through which liquid water is injected and the bottom one holds a sheathedthermocouple (type R) which measures the temperature at the sample interaction site. The fittingthat holds the capillary also has a T-fitting that can be used to introduce additional permanentgases (e.g. Ar, O 2 ) in future testing should it be necessary. The sheathed thermocouple (0.25”OD) runs through a hollow alumina tube (0.875” OD) that acts as a support for the fused quartzcapillary. Without this support the fused quartz capillary bends and breaks under its own weightat high temperatures.The capillary used in this design has an outer diameter of 3mm and inner diameter of1mm (Quartz Scientific, Inc., Fairport Harbor, OH). The capillary is cut to approximately 70cmso that when placed in the furnace, the exit opening is at the end of the hot zone of the furnace.Liquid water is pumped into the capillary using a peristaltic pump (IDEX Health & ScienceGmbH, Wertheim-Mondfeld, Germany, Ismatec Tubing Pump Model REGLO Analog) at a rateof 1.8mL/min. The water then travels through the capillary, enters the furnace, boils andaccelerates toward the sample as a gas. The gas velocity (v g ) of the water vapor was determinedusing an adaptation of the ideal gas law in the same way as dos Santos et al. 2 shown below(3)Where ṁ is the mass flow rate, R is the universal gas constant, T is the temperature of the jet, Ais the cross-sectional area of the capillary, MW is the molecular weight of water and P is the exitpressure at the end of the capillary. Using Eqn. 3 and assuming: ambient pressure = 1 atm, MW= 18.015 g/mol; with test conditions of T = 1289 °C, A = 3.141E-6 m 2 , ṁ = 1.8 g/min; a value ofv g = 272.4 m/s is obtained.

5The sample used in the test was a 1” x 1” SiO 2 slide(Quartz Scientific, Inc., Fairport Harbor, OH) that was chosento simulate the volatilization of the oxide layer shown in Eqn.2. The sample was placed in a slotted alumina boat as shown inFig. 2 and positioned in the furnace so that the gas wouldimpinge directly onto the sample. The positioning of the samplewas determined after calibration of the tube furnacetemperature profile shown in Fig. 3. Since the furnace is 22”long, it is expected that the hot zone would be in the middle atFigure 2: View down tubefurnace (Digital Camera)the 11” mark. To give the wateradditional time to boil beforeexiting the capillary, the end ofthe capillary was positioned atthe 13” mark and the slide waspositioned ½” from the end ofthe capillary.With this currentFigure 3: Furnace Calibrationconfiguration, the water exits the capillary with an audible sputtering effect in which most of thewater transforms into steam but there are sporadic bursts of liquid water through the furnace.Future tests will try to correct this issue by raising the liquid water temperature before enteringthe furnace.

6Experimental ResultsThe sample shown in these results was exposed for 170 hours at 1289°C with a constantwater flow rate of 1.8 mL/min. Unless noted otherwise, these images were taken with a digitalmicroscope (Hirox-USA, Inc., Hackensack, NJ, Model # KH 7700). The blue arrows indicate thedirection of the water jet and the red-dashed lines draw attention to material loss by outlining theoriginal sample boundaries. Fig. 3 on the right shows the orientation of the sample in the furnacefrom the perspective of looking down the tube. The quartz capillary rests on top of the aluminasupport cylinder and expels water vapor directly at the sample slide.As Fig. 4 shows, there is a distinct difference between the slide before and after the furnacetest. The sample on the right has likely crystallized into cristobalite. The interaction site betweenthe water vapor jet and sample is also evident in the top right corner of the sample. By weighingthe sample before and after the furnace run, a weight loss equal to 0.07908 g was calculated.Figure 4: Comparison of SiO 2 slide before and after exposure (Digital Camera)

7Fig. 5 gives a more detailed perspective of theinteraction site of the water vapor and the sample.The sample recession is evident and theobservable area of material loss on the front sideof the slide (outlined in green) is approximately0.5 cm 2 .This sample also showed visible recessionwhen viewed from a top perspective. As Fig. 6Figure 5: Front view of SiO 2 Slide afterfurnace exposuredepicts, the red dashed line is the approximate dimensions of the sample prior to the run and thechange in the width of the slide from the leading edge backwards is evident.Figure 6: Top view of SiO 2 slide after furnace exposureThe point of greatest recession is at the leading edge and shows a width change of 126 µm. Sincethis test was run for 170 hours, this produces a maximum recession rate of approximately 0.74µm/hr.Discussion of ResultsTo determine the accuracy of these results and the effectiveness of the device, it is necessary tocompare the results to theoretical models and predictions of material loss. Using the equationdeveloped by Robinson and Smialek 8 for SiC, which is shown below in Eqn. 4, an expectedweight loss rate (k l ) is calculated for SiO 2 (neglecting the density difference between SiO 2 and

8SiC) based on the velocity (v), temperature (T), and partial pressure (P) of the water vapor andother environmental conditions.( ) (( )) (4)By using values of T=1562 K, v = 272.4 m/s and assuming P=1 atm, this gives us a value of k 1 =0.1373 mg/(cm 2·hr). For comparison, an experimental value determined from the observedweight loss and affected area on the sample is calculated with Eqn. 5.( ) ( ) (5)Where m is mass loss, A is area of mass loss and t is time of exposure, Using input for m =0.07908 g, A = 1 cm 2 (for the two sides of the slide) and t = 170 hours; it is found that k 1 =0.4652 mg/(cm 2·hr).The calculated and measured linear weight loss rates, k, differ by a factor of 3.38 but theorder of magnitude is correct. The discrepancy may be attributed to the rough estimation of theaffected surface area and the density difference between SiO 2 and SiC which was neglected. Thiscomparison gives the semi-quantitative confirmation that the testing device provides a realisticsimulation of the water vapor flows common to combustion environments.SummaryThis furnace water jet provides an inexpensive testing device for characterizing materials’interactions with high-temperature water vapor under controlled and independently variabletemperatures and gas velocities. The results for SiO 2 show rapid and measurable materialrecession that agree with theoretical predictions. This device will enable a more comprehensiveunderstanding of the basic science behind high-velocity, high-temperature water vapor