**2.1 Plasma spraying and characterization techniques**

The surfaces of the substrate materials which are to be plasma coated were examined for dimensional accuracy and surface finish before being degreased in a vapour bath (70- 800C) of tetra chloro ethylene. The surfaces were then grit blasted by Al2O3 (-18+24 mesh) at a pressure of 455 kPa. Plasma spraying process was carried out with the help of proprietary Sulzer Metco Equipment. The composition of cast iron substrate and coating materials is given in Table 1.

Erosion Behavior of Plasma Sprayed

was taken as 20 per min.

section of the coating was reported.

**2.2 Solid particle erosion test** 

Alumina and Calcia-Stabilized Zirconia Coatings on Cast Iron Substrate 103

Microstructure analysis and surface morphology studies were carried out on a JOEL-JAPAN JSM-840A Scanning Electron Microscope. Area percentage measurements were done using a Leitz microscope fitted with a Biovis image Analyzer on the polished section of the coating. Care was taken to minimize the pull out of bond coat and top coat particles during polishing of the coated samples. The mounted samples were polished using emery papers of 240, 300, 400, 600 grit sizes and subsequently on 1/0, 2/0, 3/0 and 4/0 grades, successively. Fine polishing was done to obtain a mirror finish using 0.5 μm diamond impregnated cloth. The polished sample was cleaned with acetone before mounting on an optical microscope interfaced to a digital image capture and analysis system. The magnification was chosen such that the coating microstructure image covers the screen and allows the resolution of the voids that contributes significantly to the total porosity area percentage. The process of selecting the appropriate range of grey values was done to ensure that only voids were sampled. About ten

XRD analysis of the coated test samples were carried out on a Philips X-Ray Diffractometer (Model: PW 1840) using Cu-Kα radiation over a 2θ range of 20 to 1000. The scanning speed

For Adhesion test cast iron cylindrical substrates were prepared according to ASTM C633-79 standards. The circular face of the test sample was coated according to the procedure explained in first paragraph of this section. One more sample with the same geometry but without coating was then joined to this coated surface employing an adhesive, Epoxy Polymer 15 (EP 15) with the application of the contact pressure varying from 2-3bar. The sample was then heated to 170 0C and maintained at this temperature for more than 60 minutes, before cooling it to room temperature. The specimens thus prepared were tested in a UTM of 60 tones capacity. The maximum load, maximum tensile strength and the stress strain diagram for the specimen were displayed by the computer connected to UTM. On each sample, five tests were conducted. The microhardness of the test samples was determined using Leica Vickers Microhardness Tester (Model: VHMT Auto) as per ASTM E384 [15] standards. The test parameters are; 300 g load, 25 µg S-1 loading rate, 15 seconds dwell time with a Vickers Pyramid indenter. The measurement of hardness was done along the total thickness of the coating including substrate. An average of ten measurements taken at different locations on the transverse

Erosion tests on coated and uncoated test samples were carried out according to ASTM G76- 02 [16] standards. The test parameters are shown in Table 3. The sample was first cleaned in acetone using an ultrasonic cleaner, dried and then weighed using an electronic balance having a resolution of 0.01 mg. The sample was then fixed to the sample holder of the erosion rig and eroded with silica sand at the predetermined particle feed rate, impact velocity and impact angle for a period of about 5 min. The sample was then removed, cleaned in acetone and dried and weighed to determine the weight loss. This weight loss normalized by the mass of the silica particles causing it (i.e. testing time x particle feed rate) is then computed as the dimensionless incremental erosion rate. The above procedure was repeated till the incremental erosion rate attained a constant value independent of the mass of the erodent particles or, equivalently, of testing time. This constant value of the

separate fields of view were selected to ensure consistency in the analysis.


TC1-Top Coat 1, TC2-Top Coat 2, BC1-Bond Coat 1, BC2-Bond Coat 2

Table 1. Chemical composition of substrate and coating materials

The schematic diagrams of coating layers on cast iron substrate are shown in Fig. 1. The spray parameters for different materials are shown in Table 2.


CI-S1, CI-S2 and CI-S3 CI-S4, CI-S5 and CI-S6

Fig. 1. Schematic diagrams of coating layers with cast iron substrate (number in the bracket indicates the required thickness of each layer)


Table 2. Plasma spray parameters

Surface texture of the coated samples was examined, employing Mahr Perthometer. The coated plate of 100x100 sq. mm area was selected and it was divided into small segments of 10 x 10 sq. mm. The tracing length was about 5.6 mm for each of the selected segment. Typical parameters describing the surface quality such as Arithmetical Mean Deviation or Average Roughness (Ra), Mean Roughness Depth (Rz), Maximum Roughness Depth (Rmax), Core Roughness Depth (Rk), Reduced Peak Height (Rpk), Distance between the Highest Profile Peak and the Reference Line (Rp), Root Mean Square Deviation (Rq) were recorded on each of the segments subjected to the analysis.

Substrate material Cast Iron C-3.54, Si-2.21, Mn-0.67, Cr-0.025, Cu-0.013, P-0.056, S-0.031,Fe-balance Coating material

> Metco 452 (BC1)

The schematic diagrams of coating layers on cast iron substrate are shown in Fig. 1. The

CI-S1, CI-S2 and CI-S3 CI-S4, CI-S5 and CI-S6 Fig. 1. Schematic diagrams of coating layers with cast iron substrate (number in the bracket

> Current A

Voltage V

Spray distance mm

Feed rate kg/hr

Carrier gas (Argon) Flow lpm

TC1 700 520 60 600 65 64-125 2.7 TC2 345 345 37 500 75 50-100 5.4 BC1 700 340 37 500 65 100-175 4.1 BC2 700 340 37 500 65 100-175 4.1

Surface texture of the coated samples was examined, employing Mahr Perthometer. The coated plate of 100x100 sq. mm area was selected and it was divided into small segments of 10 x 10 sq. mm. The tracing length was about 5.6 mm for each of the selected segment. Typical parameters describing the surface quality such as Arithmetical Mean Deviation or Average Roughness (Ra), Mean Roughness Depth (Rz), Maximum Roughness Depth (Rmax), Core Roughness Depth (Rk), Reduced Peak Height (Rpk), Distance between the Highest Profile Peak and the Reference Line (Rp), Root Mean Square Deviation (Rq) were

99.5 Al2O3 ZrO2 5CaO Fe 38Ni10Al Al2O330(Ni 20Al)

Metco 410NS (BC2)

Metco105SFP (TC1)

Materials

Metco 210NS (TC2)

TC1-Top Coat 1, TC2-Top Coat 2, BC1-Bond Coat 1, BC2-Bond Coat 2 Table 1. Chemical composition of substrate and coating materials

spray parameters for different materials are shown in Table 2.

Secondary gas(H2) Pressure kPa

recorded on each of the segments subjected to the analysis.

indicates the required thickness of each layer)

Primary gas (Argon) pressure kPa

Table 2. Plasma spray parameters

Microstructure analysis and surface morphology studies were carried out on a JOEL-JAPAN JSM-840A Scanning Electron Microscope. Area percentage measurements were done using a Leitz microscope fitted with a Biovis image Analyzer on the polished section of the coating. Care was taken to minimize the pull out of bond coat and top coat particles during polishing of the coated samples. The mounted samples were polished using emery papers of 240, 300, 400, 600 grit sizes and subsequently on 1/0, 2/0, 3/0 and 4/0 grades, successively. Fine polishing was done to obtain a mirror finish using 0.5 μm diamond impregnated cloth. The polished sample was cleaned with acetone before mounting on an optical microscope interfaced to a digital image capture and analysis system. The magnification was chosen such that the coating microstructure image covers the screen and allows the resolution of the voids that contributes significantly to the total porosity area percentage. The process of selecting the appropriate range of grey values was done to ensure that only voids were sampled. About ten separate fields of view were selected to ensure consistency in the analysis.

XRD analysis of the coated test samples were carried out on a Philips X-Ray Diffractometer (Model: PW 1840) using Cu-Kα radiation over a 2θ range of 20 to 1000. The scanning speed was taken as 20 per min.

For Adhesion test cast iron cylindrical substrates were prepared according to ASTM C633-79 standards. The circular face of the test sample was coated according to the procedure explained in first paragraph of this section. One more sample with the same geometry but without coating was then joined to this coated surface employing an adhesive, Epoxy Polymer 15 (EP 15) with the application of the contact pressure varying from 2-3bar. The sample was then heated to 170 0C and maintained at this temperature for more than 60 minutes, before cooling it to room temperature. The specimens thus prepared were tested in a UTM of 60 tones capacity. The maximum load, maximum tensile strength and the stress strain diagram for the specimen were displayed by the computer connected to UTM. On each sample, five tests were conducted.

The microhardness of the test samples was determined using Leica Vickers Microhardness Tester (Model: VHMT Auto) as per ASTM E384 [15] standards. The test parameters are; 300 g load, 25 µg S-1 loading rate, 15 seconds dwell time with a Vickers Pyramid indenter. The measurement of hardness was done along the total thickness of the coating including substrate. An average of ten measurements taken at different locations on the transverse section of the coating was reported.
