Wear Analysis of Plasma Sprayed Calcium and Strontium Zirconates on Inconel 718

Abstract

CaZrO₃and SrZrO₃powders were prepared by sol-gel synthesis and plasma sprayed over the Inconel 718 substrate,forming a wear-resistant ceramic coating. X-Ray Diffraction (XRD) analysis of synthesized ceramic powders revealed the presence of CaZrO₃and SrZrO₃ phases, having an orthorhombic structure. CaZrO₃ and SrZrO₃powders were sprayed onto the surface of Inconel 718using a plasma spraying process. Scanning Electron Microscope (SEM)analysis of CaZrO₃and SrZrO₃coated samplesshowedanaveragethickness of about 400μm.Wear depth and wear mechanism for the above sampleswerestudiedusing Pin-on-disc apparatus. Wear results of CaZrO₃ and SrZrO₃ coated substrates revealed that high wear depths were observed at high speeds and loads. SEM investigations of worn-out samples revealed that both CaZrO₃ and SrZrO₃ coated samples had undergone delamination wear. CaZrO₃ coated samples had exhibitedless delamination and wear depth than SrZrO₃ coated samples, thus showing relatively better wear resistance.

Keywords:
Wear-resistant Coatings; Calcium Zirconate; Strontium Zirconate; ceramic coatings; Sol-gel synthesis; Atmospheric plasma spraying

1. Introduction

Ceramic oxide coatings are wear-resistant since they are thermodynamically stable at elevated temperatures¹. Al₂O₃-ZrO₂ has similar wear-resistant nature in addition to high fracture toughness. Zirconia subjected to formation of thermal oxide layer has high tribological wear resistance. However, specific changes in crystal structure cause deterioration in abrasion properties, limiting its applications². Deterioration is usually the presence of defects or impurities in the crystal lattice of the synthesized zirconates. These defects can weaken the material's structural integrity, making it more prone to damage from abrasive forces. Another type of change is the polymorphism of the CaZrO₃ and SrZrO₃, wherein the structural changes undergoes stability changes. For example, if the crystal lattice of the zirconate becomes distorted or disordered, this can lead to a reduction in its abrasion resistance. Additionally, if the crystal structure undergoes phase transitions at high temperatures or under other extreme conditions, this can also cause deterioration in its abrasion properties. Overall, any changes in the crystal structure of zirconates that result in a reduction in their strength, toughness, or resistance to wear²-⁴. The tribological behaviours of the coatings are assumed to be closely related to their physical and mechanical properties³-⁵. These coatings have applications in orthopedics, cutting tools, biomedical implants, articular head replacement and clinics³,⁶. High plasma power, high-velocity feed andfast deposition time give an optimal process in the case of Titanium dioxide coatings¹. In general, the ceramic coating materials were prepared by solid-state synthesis, i.e.ball milling method, but the resulting samples contain agglomerated grains of unequal sizes and often contaminated due to impurities incorporated from abrasive particles or incomplete reactions. In contrast to the powder processing methods, the liquid phase processing (e.g., solution-sol-gel technique) provides a molecular-level mixing of the individual components, which allowsthe diffusion path in the nanometer range to yield crystalline material at much lower temperatures than generally required for the solid-state reactions⁷-¹².

CaZrO₃ and SrZrO₃ are both ceramic oxides of the perovskite family¹³. The process of atmospheric plasma spraying of ceramic coating and various chemical treatments of the prepared ceramic powders were different in the way of their properties and applications from materials synthesized through other routes⁶,¹³. The atmospheric plasma sprayed ceramic systems such as calcium and strontium zirconates synthesized through chemical methods exhibited significant difference in the particulate nature especially morphologies.Hence the properties of the zirconates prepared through these methods will exhibit different properties when compared to the ceramic systems developed through other synthesis methods and as a result the coating properties will change.The plasma spray distance, plays a significant role in the formation of defect free coatings with higher coating efficiency¹⁴. The plasma-sprayed coatings using these ceramic powders show better mechanical properties such as indentation crack resistance, adhesion strength and spallation resistance³,⁴. Moreover, coatings are selected with combined properties of ductility, strength and wear resistance.

One-step Plasma Electrolytic Oxidation(PEO), micro-arc oxidation and laser cladding are some of the widelyusedprocesses for ceramic powder coatings²-⁵,¹³,¹⁵,¹⁶. Recent studies reported that atmospheric plasma spraying (APS) had got relatively high deposition efficiency, flexibility, and formation of uniform microstructures¹⁷-²⁴. During Atmospheric Plasma Spraying (APS), the ceramic powders are melted at high temperatures up to 1300ºC and sprayed over the substrate¹⁵,¹⁹-²³,²⁵,²⁶. APS is a more complicated process, which involves numerous parameters. In this present work, CaZrO₃(CZO) and SrZrO₃(SZO) were coated over the Inconel 718substrate by the APS technique.Inconel 718 is a high-strength, corrosion-resistant nickel-based superalloy that is commonly used in high-temperature and high-stress applications, such as aerospace and gas turbine engine components¹⁵,¹⁶. It has been a potential choice for coating with ceramic materials due to excellent thermal stability, making it an ideal material for use in high-temperature environments, higher resistant to corrosion and oxidation, even in hostile environments, high wear resistance, such as cutting tools and thermal barrier in various aerospace applications²²,²³. Spraying speed, distance, arc current, powder feed rateand carrier gas flow rate are a few parameters that can be altered in the APS technique³-⁵,¹⁵,¹⁶,²⁶. Coated substrates were subjected to wear tests using a pin-on-disk tribometer. Wear studies of coated materials using pin-on-disc tribometer revealed that the frictional force is higher in ceramic-metal pair than in ceramic-ceramic pair³,¹³. Coatings deposited byAPStechniquewithhigher particle speedexhibited relatively better abrasion-wear resistance than the coatings deposited by the High-velocity Oxy-fuel process, as reported byWei-Cheng Lih et al.,At higher loads and sliding velocity, the earlier effect of the fatigue and the abrasive effect are present. It is also reported that the debris impact on the worn surface of the samples leads to severe wear. Literatures reveal that primary wear mechanisms are abrasive wear, adhesive wear, severe plastic deformation, and oxidation in samples. Ceramic coatings can improve the wear resistance of metal with carbon nanotube fibers or by lasercladding, which provides a phase-strengthening effect¹⁵.

2. Materials and Methods

2.1. Sol-gel synthesis

Analytical grades of powders (99% purity-Loba chemicals): calcium chloride (CaCl₂.2H₂O), strontium chloride (SrCl₂.2H₂O), and zirconium oxychloride (ZrOCl₂.8H₂O) powders were taken in the stoichiometric ratio for the synthesis of calcium zirconate (CaZrO₃) and strontium zirconate (SrZrO₃). In addition to the above precursors, EthyleneDiamineTetraAceticacid (EDTA) 0.5N and sodium hydroxide (NaOH) 0.1M were also used for thepreparation of the compounds. A facile sol-gel methodology was adopted to prepare the CaZrO₃ and SrZrO₃ compounds respectively. The equimolar composition of calcium, strontium and zirconium salt solutions were prepared using distilled water and EDTA was added as a chelating agent. The solutions were mixed at room temperature and NaOH solution (0.1 M) was added to adjust the pH. The prepared gels were then washed with distilled water, dried at 120°C and finally calcined at 1100°C for 8 hours. The crystal structures of the synthesized CaZrO₃ and SrZrO₃ powder particles were determined using Shimadzu Spectrometer Model XRD 6000 with Cu-Kα as target. Indexing and confirmation of the phases in the powder particles were carried out using the ICDD-PDF2 database.

2.2. Thermal spraying process

The Inconel 718 substrate of dimension (60 mm x 38 mm x 6 mm) was first sand-blasted followed by a degreasing process to ensure a clean surfacebefore the coating process. The coating was prepared by APS (Air Plasma Spray) technique with the standard parameters,as shown in Table 1. Initially, the bond coat of NiCoCrAlYwas coated onto the substrate.The reason for coating a bond coat of NiCoCrAlY onto the Inconel substrate is to provide oxidation and corrosion resistance to the substrate material. The bond coat acts as a barrier layer between the substrate and the topcoat, which typically gives rise to interdiffusion of elements. Over the bond coat, CaZrO₃ powders were deposited to achieve a uniform coating thickness of homogenous composition. A similar methodology was adopted for coating SrZrO₃ over the substrate. The surface morphology of the powder-coated substrate was analyzed using a JEOL JSM-6360LV Scanning Electron Microscope (SEM).

Table 1
Plasma Spray Coating Parameters.

Parameters	Bond Coat	Top Coat 1	Top Coat 2
Current (A)	500	550	550
Voltage (V)	60	70	70
Spraying Distance (mm)	100	100	100
Spraying Speed (mm/sec)	10	10	10
Powder Feed Rate (g/min)	35	35	20
Ar/He Ratio	15/50	15/50	15/50

2.3. Wear analysis

Wear tests were conducted for CaZrO₃ and SrZrO₃ coated specimens in the Ducompin-on-disc tribometerwith a data acquisition system. Wear tests were carried out following the Design of Experiments(DOE) methodology (L27 orthogonal array). Sliding speed, load and time were chosen as parameters for wear study. In the L27 orthogonal array, three factors with 3 level designs were chosen. Samples were cut to dimensions of8mmx8mmx 6mm. The speed and time of subjection to wear were fed into the device and the load was varied by applying weights. Each experiment was performed by varying these three parameters. After the completion ofthetest, wear depth was determined. Morphology of worn-out surface was analyzed using JEOL JSM-6360LV Scanning Electron Microscope (SEM).

3. Results and Discussion

3.1. XRD analysis of synthesized CaZrO₃ and SrZrO₃ powders

The ICDD-PDF2 database was used to analyze the XRD pattern obtained for the CaZrO₃and SrZrO₃ powder samples. The XRD patternofcalciumzirconate,as shown in Figure 1a, indicates the presence ofpure CaZrO₃ phase, which has an orthorhombic form of crystal structural structure. The lattice parameter of obtained phase was(a = 5.7616 Å, b = 8.0171 Å, c = 5.5912 Å, α = β = ƴ = 90°) with space group of Pbmn. Crystallite size was determined from X-ray line broadening using Debye Scherer’s formula²⁶and was found to be 4.80nm.

Figure 1
XRD pattern of synthesized CaZrO₃(a) and SrZrO₃(b).

The XRD pattern of SrZrO₃ powder-coated substrate as shown in Figure 1b, reveals the presence of pure SrZrO₃phase with a space group of Pbnm lattice parameters of (a = 5.7862 Å, b = 5.8151 Å, c = 8.196 Å, α = β = ƴ = 90°). The crystallite size was determined from X-ray line broadening using Debye Scherer’s formula²⁶ and found to be 5.20nm.

3.2. SEM analysis of CaZrO₃ and SrZrO₃coated samples

Figures 2a and 2b showed the SEM images of synthesized CaZrO₃ and SrZrO₃ samples. It was observed from the SEM micrograph the particles sizes were non-spherical and agglomerated with indistinctive sizes among the powders. The average particle sizes were measured for both CaZrO₃ and SrZrO₃ samples at 500 X magnification as 98μm and 93μm respectively.

Figure 2
SEM analysis ofCaZrO₃ (a) and SrZrO₃ (b) synthesized powders.

Figure 3a shows the SEM image of as-sprayed CaZrO₃ on the Inconel 718 substrate. It can berevealed that the coating obtained was having an average thickness of 400.33μm. It is also evident from the SEM micrograph that inter-diffusion had occurred between top-coat and bond coat, which leads to strong bonding between them. SEM image of SrZrO₃ coated samples shown in Figure 3b, revealed that the coating developed over the substrate was having an average thickness of 400 μm.

Figure 3
SEM analysis of CaZrO₃ (a) and SrZrO₃ (b) coated substrate.

3.3. Wear behaviour ofCaZrO₃ coated sample

Wear results obtained from wear tests of CaZrO₃ coated samples are shown in Table 2. It can be revealed thatthe wear depth was higher at high-applied loads, higher speeds and higher sliding times.

Table 2
Wear test results of CaZrO₃ coated samples.

S.No.	Load(N)	Speed(rpm)	Time(minutes)	Wear depth(µm)	Coefficient of friction(µ)
1	10	300	5	92	0.75
2	10	300	10	112	0.51
3	10	300	15	137	0.49
4	10	400	5	120	0.53
5	10	400	10	136	0.57
6	10	400	15	159	0.58
7	10	500	5	140	0.46
8	10	500	10	164	0.46
9	10	500	15	83	0.47
10	15	300	5	98	0.91
11	15	300	10	123	0.91
12	15	300	15	117	0.89
13	15	400	5	132	0.92
14	15	400	10	146	0.80
15	15	400	15	123	0.72
16	15	500	5	187	0.64
17	15	500	10	219	0.63
18	15	500	15	212	0.63
19	20	300	5	102	0.43
20	20	300	10	132	0.40
21	20	300	15	157	0.38
22	20	400	5	165	0.34
23	20	400	10	176	0.32
24	20	400	15	188	0.30
25	20	500	5	162	0.33
26	20	500	10	201	0.30
27	20	500	15	233	0.30

Regression equation {Wear (µm) = - 54.9 + 4.14 Load (N) + 0.295 Speed (rpm) + 2.34 Time (minutes)} developed for the analysis of wear for CaZrO₃ samples revealed that load is the primary factor in affecting the wear of CaZrO₃ coated samples followed by sliding time and sliding speed. It can be inferred from Figure 3, that the CaZrO₃ coated sample had attained uniform top coating with fewer asperities. The sound interface achieved in the CaZrO₃ coated sampleindicated that an excellent bonding had taken place between the top-coat and substrate.CaZrO₃ coated sample exhibited excellent wear resistance at lower loads even though sliding speed and time were increased. Wear studiesindicated that at higher loads and sliding speeds, high compressive forces developed between the asperities of coated sample and disc, caused localizedfragmentation of asperities.This resulted in formation of debris and led to severe wear of coated samples. Wear depth was also found to increase with increasing sliding time and sliding speed.

Figures 4, 5 and 6 revealed that CaZrO₃ coated samples had undergone mild delamination at lower load, lowerslidingspeed and lower sliding time. It can also be observed that mild delamination is transformed into severe delamination at higher loads, speeds and sliding times. It can also be understood from Figures 4, 5 and 6 that at constant sliding speed and sliding time,wear depth wasfound to increase with an increase in load. It is also confirmed that at lower loads, sliding speeds and sliding time of CaZrO₃ coated samples had exhibited adhesive wear mechanism. At higher loads, sliding speeds and sliding time CaZrO₃ coated samples have undergone the transition from adhesive wear to abrasive wear.

Figure 4
Worn-out surfaces of CaZrO₃ coated samples at (a) 10N,300 rpm and 5 minutes (b) 15N,300 rpm and 5minutes.

Figure 5
Worn-out surfaces of CaZrO₃ coated samples at (a) 15N,400 rpm and 10 minutes (b) 20N,400 rpm and 10minutes.

Figure 6
Worn-out surfaces of CaZrO₃ coated samples at (a) 15N,500 rpm and 15 minutes (b) 20N,500 rpm and 15minutes.

The main effects plot in Figure 7 also revealed that load influenced the wear depth of CaZrO₃ coated samples to a larger extent followed by sliding speed. However,with an increase in sliding time wear depth was increasing initially and it was constant after the initial increase in wear depth. This indicated that generation of debris from fragmentation of asperities at higher load and sliding speed decreased aftera certain sliding time.

Figure 7
Main Effects plot for means of wear values of CaZrO₃ coated samples.

3.4. Wear behaviour for SrZrO₃coated sample

Wear results obtained from the wear test of SrZrO₃samples are shown in Table 3. SrZrO₃ coated samples also showed higher wear depths at higher load, higher speed and higher sliding time. However, wear depthwas found to be higher than CaZrO₃ coated samples because of the large number of asperities formed on the topcoat layer.

Table 3
Wear results of SrZrO₃ coated samples.

S.No.	Load (N)	Speed (rpm)	Time (minutes)	Wear depth (ìm)	Coefficient of friction(ì)
1	10	300	5	147	0.73
2	10	300	10	153	0.71
3	10	300	15	164	0.65
4	10	400	5	166	0.61
5	10	400	10	189	0.55
6	10	400	15	214	0.41
7	10	500	5	178	0.49
8	10	500	10	193	0.45
9	10	500	15	229	0.44
10	15	300	5	181	0.68
11	15	300	10	197	0.65
12	15	300	15	207	0.62
13	15	400	5	243	0.69
14	15	400	10	267	0.65
15	15	400	15	294	0.61
16	15	500	5	274	0.57
17	15	500	10	282	0.55
18	15	500	15	309	0.53
19	20	300	5	211	0.42
20	20	300	10	228	0.40
21	20	300	15	246	0.36
22	20	400	5	299	0.33
23	20	400	10	341	0.32
24	20	400	15	387	0.32
25	20	500	5	426	0.30
26	20	500	10	450	0.28
27	20	500	15	468	0.27

Regression equation for the analysis of wear of SrZrO₃ coated samples {Wear (µm) = - 303 + 9.62 Load (N) + 0.884 Speed (rpm) + 6.47 Time (minutes)} also indicated that wear depths are higher at higher loads, higher speed and higher sliding time.

Figures 8, 9 and 10 reveal that SrZrO₃ coated samples had undergone severe delamination than CaZrO₃ coated samples at all levels of load, sliding speed and sliding time. This severe delamination can also be attributed to formation of higher amount ofdebrisinthecontactregion.At higher levels ofload,speed and time due to the formation of higher amount of localized debris,wear depth was much severe than the wear of CaZrO₃ coated samples. In SrZrO₃ coated samples also, wear depth increased with an increase in load at constant sliding speed and sliding time as indicated in Figures 8, 9 and 10. SrZrO₃ coated samples also had undergone transition from adhesive wear to abrasive wear at higher loads, sliding speeds and sliding time.

Figure 8
Worn-out surfaces of SrZrO₃ coated samples at (a) 10N,300 rpm and 5 minutes (b) 15N,300 rpm and 5minutes.

Figure 9
Worn-out surfaces of SrZrO₃ coated samples at (a) 15N,400 rpm and 10 minutes (b) 20N,400 rpm and 10minutes.

Figure 10
Worn-out surfaces of SrZrO₃ coated samples at (a) 15N,500 rpm and 15 minutes (b) 20N,500 rpm and 15minutes.

The main effects plot of SrZrO₃coated samples in Figure 11 indicated that load had got major influence in affecting the wear resistance of the samples. The wear depth of the SrZrO₃ coated sample increased with an increase in sliding time due to the formation of a large number of debris compared with CaZrO₃ coated samples.

Figure 11
Main Effects plot for means of wear values of SrZrO₃ coated samples.

4. Conclusion

The synthesized powders of Calcium zirconate and Strontium zirconate were coated ontheInconel 718 substrate with an intermediate layer of NiCoCrAlY as a bond coat.SEM analysis of CaZrO₃ and SrZrO₃coated samples revealed that the coating was consistent in terms of thickness. SEM analysis also revealed that CaZrO₃ coating was much more uniform with fewer asperities and SrZrO₃ coating was non-uniform with numerous asperities. Wear behaviour of CaZrO₃ and SrZrO₃ coated samples revealed that the wear rate of the SrZrO₃sample was comparatively higher than the CaZrO₃ coated sample. It was evident that the wear rate increases with an increase inload and speedfor CaZrO₃and SrZrO₃coatedsamples due to the formationofasperities. SEM investigations of worn-out samples indicated that both samples exhibited delamination wear mechanisms. SEM investigations of worn-out samples indicated that both CaZrO₃ and SrZrO₃ samples exhibited a transition from adhesive to abrasive wear mechanism at higher loads, sliding speeds and sliding time.However, the CaZrO₃ coated sample had exhibited less delamination & wear rate than the SrZrO₃ coated samples because localized asperities and existing asperities in the SrZrO₃ coated sample had caused relatively higher wear and delamination than CaZrO₃ coated sample.

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Publication Dates

• Publication in this collection
  14 June 2024
• Date of issue
  2024

History

• Received
  15 June 2022
• Reviewed
  03 Mar 2023
• Accepted
  25 Sept 2023