**Abstract**

SS316 is a commercial stainless steel. MTBF (Mean Time Between Failure) of SS 316 wear prone areas can be effectively increased by ceramic coating. The coating thickness, surface roughness, coating microhardness, abrasion rate, and coating porosity decides the quality and durability in ceramic coating. The current research work explains an experimental investigation to optimize the Atmosphere Plasma Spray process input parameters of Al2O3-40%TiO2 ceramic coatings. Threelevel L18 Orthogonal Array (OA) design of Experiments (DoE) is used to conduct the current work. The main input parameters considered in the current study are nozzle distance, substrate speed, arc current, carrier gas flow, and coating powder flow rate. The output parameters considered are coating thickness, surface roughness, coating microhardness, abrasion rate, and percentage of porosity. Mathematical models are generated for individual output parameters. AHP (Analytical Hierarchy Process) is effectively used to find out weights for individual output parameters treating them as objective functions, and a combined objective function is generated.

**Keywords:** atmospheric plasma spray (APS) coating, SS316, teaching learning based optimization (TLBO), Al2O3-40%TiO2

#### **1. Introduction**

To achieve increased reliability and performance of damage prone industrialrelated components, surface engineering is hugely now applied using large field of new technologies. The quest for higher efficiency and productivity across the entire spectrum of manufacturing and engineering industries has ensured that most of the machine components are subjected to highly harsh environments during routine

operation [1, 2]. The high deterioration of parts and their ultimate failure has been traced to material damage bought in by hostile environments like high relative motion between mating services, corrosive media, extreme temperatures, and cyclic stresses. As a result of the above, the concept of applying engineered surfaces capable of combating the high degradation phenomena like wear, corrosion, and fatigue to improve component performance, reliability, and life cycle has gained high acceptance in last one decade. To act as a last line of defense, a proactive coating deposited to act as a perfect barrier between the initial surface of the component and the aggressive environment that is exposed during routine operation is now globally acknowledged as an attractive and effective solution to significantly reduce damage.

Major input parameters of a plasma arc spray are below:

*An Experimental Investigation of Al2O3-40% TiO2 Powder Amalgamated…*

3.Electric power input/Arc current (250–600 Amps)

11.Pre-spray sand blasting particle size (20/24/60 μm)

12.Bond coat material type (Ni Cr 80/20, 60/40, 70/30)

17.Gas injection angle and parameters related with gun

20.Use of device for coating (Manipulator/robot/manual)

22.Distance between the substrate and the nozzle (75–125 mm)

21.RPM of the job, while coating (100–500 RPM)

The major output parameters are listed below:

15.Spray gun Coolant type (Chilled water or normal water, air)

18.Nozzle diameter (6, 6.5, 7, 7.5, 8 mm, GP, GH, GE, G Profiles)

16.Mixing combination of Al2O3/TiO2 (13% Wt TiO2, 40% Wt TiO2, 20%Wt

2.Powder feed rate (19–56 g/min)

*DOI: http://dx.doi.org/10.5772/intechopen.92175*

5.Spray distance (70–200 mm)

7.Composition of working gas

8.Fuel gas to oxygen ratio

9.Powder particle size

10.Powder morphology

13.Post spray treatment

19.Sealing after coating

1.Porosity %

**85**

2.Coating thickness

TiO2)

14. Substrate temperature (40–200°C)

6.Flame temperature (14000–16,000°C)

4. Substrate rpm

1.Carrier gas flow rate (Ar: 20–45 L/min, H2: 8–14 L/min)

316 stainless steel is widely used in chemical/petrochemical industry, food processing, pharmaceutical equipment manufacturing, medical devices fabrication, in potable water/wastewater treatment/marine applications and architectures. SS316 composition consists of Chromium 10–14%, Nickel 2–3%, carbon 16–18%, and Molybdenum. In general, the addition of about 2% Molybdenum in SS316 stainless steel provides excellent level of resistance against pitting corrosion and stress corrosion cracking especially in a saline environment compared to SS304. Abrasion and wear resistance is the type of damage which needs to be improved during its application in industrial environment. Abrasion resistance capabilities of SS316 can be highly enhanced by providing a coating of Al2O3-40%TiO2 on an SS316 steel surface by means of atmospheric plasma spraying.

Hence Al2O3-40%TiO2 coating on SS316 surface makes it an ideal combination to combat today's highly drastic and hostile industrial environments where assets are sweating to achieve target of productivity and yield which is always more than 100% of the rated capacity. Even though there is a high level of interest and expectations regarding the fundamentals and development of atmospheric plasma arc spraying, there is a huge gap of reliable as well as dependable models that correlates final engineering properties of coatings such as surface roughness, microhardness, porosity, etc. with variations in critical input process parameters and geometry of deposition process [3].

In the present work, Al2O3-40% TiO2 is coated on SS316 substrates and all the desired critical output parameters are measured and recorded for further analysis.

## **2. Experimentation details**

Justification for the usage of Al2O3-40% TiO2 amalgamated powder.

Al2O3-40% TiO2 amalgamated powder is used for all the experiments conducted during this research work. The use of Al2O3-40% TiO2 justified as it helps to decrease the melting temperature of amalgamated composite powder and hence the final coating exhibits very low porosity % and enhanced fracture toughness compared to 97/3 and 87/13 Al2O3 and TiO2 combinations. The coating generated with Al2O3-40% TiO2 amalgamated powder also possesses high dielectric strength, enhanced wear and heat resistance [4].

#### **2.1 Development of experimental plan**

As per the literature review, the following process parameters play a deciding role in the Al2O3-TiO2atmospheric plasma spray process on various substrates.

*An Experimental Investigation of Al2O3-40% TiO2 Powder Amalgamated… DOI: http://dx.doi.org/10.5772/intechopen.92175*

Major input parameters of a plasma arc spray are below:


operation [1, 2]. The high deterioration of parts and their ultimate failure has been traced to material damage bought in by hostile environments like high relative motion between mating services, corrosive media, extreme temperatures, and cyclic stresses. As a result of the above, the concept of applying engineered surfaces capable of combating the high degradation phenomena like wear, corrosion, and fatigue to improve component performance, reliability, and life cycle has gained high acceptance in last one decade. To act as a last line of defense, a proactive coating deposited to act as a perfect barrier between the initial surface of the component and the aggressive environment that is exposed during routine operation is now globally acknowledged as an attractive and effective solution to

316 stainless steel is widely used in chemical/petrochemical industry, food

Hence Al2O3-40%TiO2 coating on SS316 surface makes it an ideal combination to combat today's highly drastic and hostile industrial environments where assets are sweating to achieve target of productivity and yield which is always more than 100% of the rated capacity. Even though there is a high level of interest and expectations regarding the fundamentals and development of atmospheric plasma arc spraying, there is a huge gap of reliable as well as dependable models that correlates final engineering properties of coatings such as surface roughness, microhardness, porosity, etc. with variations in critical input process parameters

In the present work, Al2O3-40% TiO2 is coated on SS316 substrates and all the desired critical output parameters are measured and recorded for further

Justification for the usage of Al2O3-40% TiO2 amalgamated powder.

during this research work. The use of Al2O3-40% TiO2 justified as it helps to decrease the melting temperature of amalgamated composite powder and hence the final coating exhibits very low porosity % and enhanced fracture toughness compared to 97/3 and 87/13 Al2O3 and TiO2 combinations. The coating generated with Al2O3-40% TiO2 amalgamated powder also possesses high dielectric strength,

Al2O3-40% TiO2 amalgamated powder is used for all the experiments conducted

As per the literature review, the following process parameters play a deciding role in the Al2O3-TiO2atmospheric plasma spray process on various substrates.

processing, pharmaceutical equipment manufacturing, medical devices fabrication, in potable water/wastewater treatment/marine applications and architectures. SS316 composition consists of Chromium 10–14%, Nickel 2–3%, carbon 16–18%, and Molybdenum. In general, the addition of about 2% Molybdenum in SS316 stainless steel provides excellent level of resistance against pitting corrosion and stress corrosion cracking especially in a saline environment compared to SS304. Abrasion and wear resistance is the type of damage which needs to be improved during its application in industrial environment. Abrasion resistance capabilities of SS316 can be highly enhanced by providing a coating of Al2O3-40%TiO2 on an SS316 steel surface by means of atmospheric plasma

significantly reduce damage.

*Assorted Dimensional Reconfigurable Materials*

and geometry of deposition process [3].

enhanced wear and heat resistance [4].

**2.1 Development of experimental plan**

**2. Experimentation details**

spraying.

analysis.

**84**


The major output parameters are listed below:

1.Porosity %

2.Coating thickness


#### **Table 1.**

*Input parameters with three levels.*


10.Wear resistance

11.Dielectric strength

After considering the critical requirements of the industry, review of facilities available and thorough literature survey, the following input parameters with three levels are considered for experiments as shown as **Table 1**.

**Number of parameters**

> **Number of levels**

**87**

2

2

3

4 L16

5

**Table 2.** *Array selector for DoE.*

 L25

 L25

 L25

 L25

 L25

 L50

 L50

 L50

 L50

 L50

 L50

 L16

 L16

 L16

 L32

 L32

 L32

 L32

 L32

 L9

 L9

 L9

 L18

 L18

 L18

 L18

 L27

 L27

 L27

 L27

 L27

 L36

 L36

 L36

 L36

 L36

 L36

 L4

 L4

 L8

 L8

 L8

 L8

 L12

 L12

 L12

 L12

 L16

 L16

 L16

 L16

 L32

 L32

 L32

 L32

 3

 4

 5

 6

 7

 8

 9

 10

 11

 12

 13

 14

 15

 16

 17

 18

 19

*An Experimental Investigation of Al2O3-40% TiO2 Powder Amalgamated…*

*DOI: http://dx.doi.org/10.5772/intechopen.92175*

Similarly, the output parameters selected are: coating thickness, surface roughness, microhardness, abrasion rate, and porosity %.

### **3. Design of experiments**

Orthogonal array experimental design proposed by Taguchi can be efficiently used to examine the effect of different input parameters on the critical performance characteristics linked output parameters through compact set of experiments [5]. In-depth understanding of the process, including the minimum, maximum, and current value of the parameter is required to decide the set of input parameters that are highly affecting a process as well as the levels at which these parameters should be varied [6].

After thoroughly understanding the number of input parameters and the number of levels a proper compact can be finalized using the array selector as shown in **Table 2**, by looking at the column and row corresponding to the number of parameters and number of levels.

An L18 orthogonal array is used to carry out all the experiments as shown in **Table 3** considering the five input parameters with three levels. **Table 4** shows the complete experiment plan.



**Table 2.**

*Array selector for DoE.*

**87**

3.Microhardness

*Input parameters with three levels.*

**Table 1.**

5.Surface roughness

7.Microhardness

9.Bonding strength

10.Wear resistance

11.Dielectric strength

**3. Design of experiments**

parameters and number of levels.

complete experiment plan.

be varied [6].

**86**

6.Oxidation on surface

4.Abrasion resistance (abrasion rate)

*Assorted Dimensional Reconfigurable Materials*

8.Abrasion resistance, abrasion rate

levels are considered for experiments as shown as **Table 1**.

ness, microhardness, abrasion rate, and porosity %.

After considering the critical requirements of the industry, review of facilities available and thorough literature survey, the following input parameters with three

**No Parameter Low level Middle level High level** Spray distance of gun, mm 75 100 125 Carrier gas flow, lit./min. 20 30 50 Powder flow rate, g/min 25 35 50 RPM of the substrate 150 250 350 Arc current, A 350 400 500

Similarly, the output parameters selected are: coating thickness, surface rough-

Orthogonal array experimental design proposed by Taguchi can be efficiently used to examine the effect of different input parameters on the critical performance characteristics linked output parameters through compact set of experiments [5]. In-depth understanding of the process, including the minimum, maximum, and current value of the parameter is required to decide the set of input parameters that are highly affecting a process as well as the levels at which these parameters should

After thoroughly understanding the number of input parameters and the number of levels a proper compact can be finalized using the array selector as shown in **Table 2**, by looking at the column and row corresponding to the number of

An L18 orthogonal array is used to carry out all the experiments as shown in **Table 3** considering the five input parameters with three levels. **Table 4** shows the

#### *Assorted Dimensional Reconfigurable Materials*


**4. Experimental steps**

*DOI: http://dx.doi.org/10.5772/intechopen.92175*

conduct the Abrasion test [7].

the plasma coating process up to 110°C [13].

wire cutting as well as slow speed grinding.

**Figure 1.**

**89**

*Coating facility arrangement.*

constant during the experiment are

Al2O3-40%TiO2 powder from H C Starck, USA is utilized for coating for all the set of experiments. SS316 substrates are prepared with 27.5 mm diameter and 3 mm thickness plates and pieces of size 75 25 12 mm are prepared for coating so as to

Each experiment is carried out with three substrate samples. The substrate samples are assembled in one specially fabricated cartridge [8]. To avoid nonuniformity in thickness, the substrate samples are ground to achieve relatively good finish. Later, sand blasting is carried out on the surface of all substrate samples to ensure proper removal of oxides and other impurities [9, 10]. For sand blasting, fused alumina of grit size 60 μm from Carborandum Universal is used [11, 12]. Al2O3-40% TiO2 powder is deposited on the substrates by using a controlled atmospheric plasma stray system of Metco USA through an SG 100 model Plasma Gun. To ensure the removal of moisture, the amalgamated powder is preheated before

*An Experimental Investigation of Al2O3-40% TiO2 Powder Amalgamated…*

As pre the L18 orthogonal array DoE, the experiments are conducted. The coating facility arrangement is shown in **Figure 1**. The parameters which are kept

Spray nozzle GP, diameter: 5.43 mm Grind blasting pressure 2 kg/cm<sup>2</sup> Substrate exposure to gun 30 s Primary gas pressure 100 Psi Secondary gas pressure 80 Psi

The substrate samples are cleaned after the coating with ethanol and properly dried to eliminate accumulation of moisture [14]. As per ASTM B499-92014 [15], coating thickness is measured. An ultrasonic thickness gauge is used for this purpose. Using Mitutoyo surface roughness tester SV-C3100, surface roughness is measured as per ASTM D127 2013 [16]. The surface testing probe is applied on the coating surface for a length of 15 mm with a pitch of 0.001 mm. The scanning speed is kept constant at 2.0 mm/s. Microhardness is measured as per ASTM B 578-872,015 [17] by Metatech MVH-1. To measure porosity as well as microhardness, the samples are sectioned by

Later, molds are prepared using Bain mount-3 molding machine supplied by Chennai Matco. The substrate surfaces on the mold are polished with emery papers

#### **Table 3.**

*L18 orthogonal array.*


#### **Table 4.**

*Complete plan of experiments as per L18 OA.*

*An Experimental Investigation of Al2O3-40% TiO2 Powder Amalgamated… DOI: http://dx.doi.org/10.5772/intechopen.92175*
