Oxidation Behavior of Inconel 625 Alloy through Aliminide Diffusion Method Using Y2O3 and ZrO2 Nanoparticles

as a source of cobalt, NH 4 Cl as a source of activator; nano Y 2 O 3 -ZrO 2 as a source of reactive element oxide; and Al 2 O 3 as a source of filler metal. The process was carried out for 6 hours at 1100 o C temperature. The microstructure characterization of the coating was performed by SEM, EDS, and XRD. It was found that the cross-section of the coating obtained was uniform and free from cracking. The maximum hardness value was found at the outer layer (997H.V.) and decreased toward the core sample core (366H.V.). The coating's microstructure consists of an outer layer, a transition layer, and an IDZ. The average coating thickness is 132.37, 36.11, and 37.65µm for the outer layer, transition layer, and IDZ, respectively. The XRD analysis of the coating system after 6 hours at 1100 o C revealed phases formed by AlNi 3 , CoO, Al-Cr-Co, and Cr 4 NiZr. The n (growth rate time constant) and Kp (parabolic rate constant) values increase with increased oxidation temperature. It was found that adding Zr and Y to the Cr-Co-modified aluminide coating might increase the oxidation resistance.


H I G H L I G H T S A B S T R A C T
• Nano Y2O3 and ZrO2 particles were used in diffusion coating by the single-step pack cementation process • Effects of Y2O3 and ZrO2 on mechanical properties and oxidation resistance of coated Inconel 625 superalloy were analyzed • Microstructure of coated IN625 superalloy consists of an outer layer, a transition layer, and an interdiffusion zone • Adding Y2O3 and ZrO2 to the Cr-Comodified aluminide coating might increase the oxidation resistance The pack cementation process was used to create a type of Y2O3+ZrO2 doped Cr-Co-modified aluminide coating that takes advantage of the synergistic effects of nano Y2O3 and ZrO2 particles. A Ni-based superalloy (type IN625 type) was coated with pack powder containing: Al as a source of aluminum; Cr as a source of chromium, Co as a source of cobalt, NH4Cl as a source of activator; nano Y2O3-ZrO2 as a source of reactive element oxide; and Al2O3 as a source of filler metal. The process was carried out for 6 hours at 1100 o C temperature. The microstructure characterization of the coating was performed by SEM, EDS, and XRD. It was found that the cross-section of the coating obtained was uniform and free from cracking. The maximum hardness value was found at the outer layer (997H.V.) and decreased toward the core sample core (366H.V.). The coating's microstructure consists of an outer layer, a transition layer, and an IDZ. The average coating thickness is 132.37, 36.11, and 37.65µm for the outer layer, transition layer, and IDZ, respectively. The XRD analysis of the coating system after 6 hours at 1100 o C revealed phases formed by AlNi3, CoO, Al-Cr-Co, and Cr4NiZr. The n (growth rate time constant) and Kp (parabolic rate constant) values increase with increased oxidation temperature. It was found that adding Zr and Y to the Cr-Co-modified aluminide coating might increase the oxidation resistance.

Introduction
A common material for gas turbine applications is the nickel-based superalloy Inconel 625 (IN625), which has good hightemperature strength, creep resistance, corrosion resistance, and great weldability. Diffusion coating was created to protect the substrates at high temperatures so that superalloys may maintain their exceptional qualities for a longer period while working, improving properties, and raising operating temperatures [1,2,3]. It is required to modify the surface to increase oxidation and corrosion resistance. The corrosion and oxidation resistance of IN625 has been improved by efforts involving protective film coating. One technique to boost IN625's resistance to oxidation is to form an intermetallic compound, like nickel aluminide, that covers the entire surface. As a result of the nickel aluminide layer's role as an aluminum sink, alumina film can grow at high temperatures and serve as a barrier against oxidation and oxygen penetration [4,5,6]. The newer, maximum-strength Nibased superalloys, however, typically have low oxidation resistance [6]. Thus, the application of exterior protective coatings enhances the oxidation resistance of turbine blades and vanes. There are numerous ways to increase the amount of aluminum at the surface, which causes nickel aluminide to develop on the surface of IN 625 [7]. The so-called pack cementation process, used for aluminide nickel, is a recently developed procedure.
It has been standard procedure to coat or enrich the surface of high-temperature alloys with aluminum since the preceding decade to increase their resistance to environmental effects by producing an outer, protective alumina layer or scale. However, Al, Cr, Co, or other reactive element dopant integration attempts in vapor-processed aluminide coatings have been unsuccessful [8,9,10]. 861 The advantages of Y, Zr, Ce, Co, Hf, and Si-modified aluminide coatings have been extensively researched in recent years. Researchers have previously established that Cr-modified aluminide coatings have strong oxidation resistance and hot corrosion resistance. They have also researched Co, Y, Ce, and Y-Ce co-modified aluminide coatings and discovered that the inclusion of the elements can improve hot corrosion resistance. However, little research has been done to date on the microstructure and oxidation resistance of the Co-Cr-Y2O 3 -modified aluminide coating [11,12].
The main factors affecting an aluminide coating's structural stability at high temperatures are (i) processes of selective oxidation and hot corrosion, and (ii) inter-diffusion between the coating and the substrate. There has not been a thorough investigation into the impact of aluminide coatings on the substrate of IN625 superalloy doped with Y and Zr, two rare earth elements [13,14,15]. This study looked at how adding Y 2 O 3 and ZrO 2 in modest amounts at high temperatures affected the aluminide-coated IN625 substrate.
The aim of this work attempt using Nano ZrO 2 and Y 2 O 3 particle coating to form new diffusion aluminide layers onto IN625 alloy substrates and also study the effect of Nanoparticle addition on the oxidation resistance of coated and uncoated samples.

Materials
In this study, (IN625) was employed as rod specimens with dimensions of 16 mm in diameter and 3 mm in thickness. Table 1 shows the stated chemical compositions from the manufacturers.

Preparation of Samples for Experimental Procedures
The samples were degreased in alcohol and cleaned with an ultrasonic cleaner before being ground on SiC. paper up to 800. Then, the samples were polished by using a special cloth and alumina solution with a particle size of 0.3 µm. The etching process was carried out using a mixture of acids (three parts hydrochloric acid, two parts nitric acid, two parts dis. Water, and one part glycerol) immersing the surface of the sample into this solution for (10-30)sec.

Pack Cementation Coating
The pack cementation procedure was used to apply the aluminide diffusion coating at 1100°C for 6 hours while in an Ar environment. Pack powder consists of 0.5wt.% ZrO 2 -0.5wt.% Y 2 O 3 , 15wt.%Al,8wt.% Cr,7wt.%Co, 5wt.%NH 4 Cl, and Bal. Al 2 O 3 . Before combining with additional powders, the NH 4 Cl was first dried for 24 hours at 75 degrees Celsius. The particles were then mechanically mixed with additional regular hexane for a further five hours. The coating technique shown in Figure  1(a and b). was used in this work. Dry the powder combination for 10 hours at 90°C in the oven. Utilizing optical and scanning electron microscopy, the coating's microstructure and cross-section were studied. A Cu-K radiation-based X-ray "diffraction" instrument of type XRD-6000 was used to identify the phases that formed before and after coating. The x-ray diffraction was carried out at the Nanotechnology and Advanced Materials Research Center/University of Technology.

Cyclic Oxidation Test
The oxidation test was conducted in a furnace at 900°C for 50 hours, five hours at a time. The samples were taken out of the furnace and left to cool to room temperature before being weighed. The oxidation test was then finished by returning them to the furnace. These specimens' mass increase was calculated using a precise analytical balance.

Coating Microstructure
The microstructure of nano Y 2 O 3 and ZrO 2 particle doped Cr-Co modified aluminide diffusion coating is depicted in Figure 2 (a and b) at 1100 o C for 6hr. It is important to obtain that our studies' aluminide coating formation requirements (time, temperature) appear to be lower than those for traditional pack aluminizing on a Ni-base superalloy, which is typically above 760°C this match with [16,17]. The microstructure of the coating is composed of the outer layer, transition layer, and interdiffusion zone (IDZ). The transition layer is a narrow area containing some dispersed refractory materials. The columnar phase made up of this region was spread throughout the initial base material and formed mostly in the direction of diffusion. Later, this base material became richer in Al and less so in Ni. The precipitate which found at the interface of IDZ is rich in Cr, and the bottom area of IDZ is about the columnar phase. Figure 3 (a and b) demonstrates the EDS mappings are carried out to analyze the composition and concentration elements of coating and substrate for two coating systems. The EDS element analysis of the cross-section indicates that the major body of the Al-Cr-Co coating is located in the outer region. It has a similar composition to an aluminum coating and contains sizable amounts of both nickel and aluminum.
However, because chromium is less soluble in the aluminum-nickel complex, this area has more of an element than the transition region because it is added to the packed powder [18]. Figure 4 represent the XRD analysis of the coating system at 1100 o C for 6hr with different concentration of nano Y2O3 and ZrO2 particle (0.5%). The results showed a diffraction model that needs a lot of care in the analysis because the compounds formed during the coating process match more than one peak. This is what exists with the nickel-rich (Al-Ni) phase, whose composition is very close to AlNi 3 . Ni outward diffusion can also cause a diffusion zone between the coating and substrate with the substrate elements scattered from the surface of the substrate. The concentration of Ni in the coating was substantially increased, and that of Co was noticeably decreased when a little amount of Y 2 O 3 and ZrO 2 was dispersed finely in the coating [19,20]. the average coatings thickness is 132.37, 36.11, and 37.65µm for the outer layer, transition layer, and IDZ, respectively.

EDS Analysis
Aluminum diffused from the outer side to the base metal more quickly than other elements did in the other direction, particularly in the Ni-Cr-Co diffusion pair. Figure 3 shows the unique uphill diffusion of Mo, Cr, Ta, and Nb in Ni-Cr/Co diffusion couples. The peak value of Mo content was about 5%, and Figure 5 illustrates the Ni content decline in the same region, as shown in Figure 3. The phase structure can be used to explain the uphill diffusion; locally enriched precipitates of Mo, Cr, Ta, and Nb were the cause of this enrichment. It was important to note that the Cr and Co concentrations were located much farther to the right than other peaks. Figure 5 shows the coated specimen treated at 1100°C for 6 hours' composition distribution. The aluminide coatings were composed of two layers. The uppermost layer is composed of coarse grains with significant Ni and Al concentrations and is about 132µm thick. It can be inferred that Cr-Zr and Al-Cr-Co intermetallics were not abundant in the coating's top layer, which was dominated by Al-rich phases like NiAl 3 . The coating thickness was approximately 37µm. It was dense, homogeneous, and possessed columnar crystals with coarse and fine grain, as well as precipitates that were common in aluminide coatings on Ni-base superalloys created using the pack aluminide process. The element's content altered considerably along the IDZ-outer layer interface. The IDZ was composed of an AlNi3 matrix including different precipitates.
According to EDS analysis as seen in Figures (3 and 5), the coating had a gradient distribution across it and contained all of the elements in the substrate alloy in relative proportions matching their concentrations in the alloy. Due to the dilution of aluminum in the coating, these elements' absolute concentrations were lower than those of the substrate [21]. Below the coating's substrate, chromium was found in significant amounts. The most likely explanation for this was that chromium had been forced into the interface due to its low solubility in the matrix (AlNi3), which created a Cr 4 NbZr barrier layer to prevent aluminum diffusion into the substrate.  Figure 6 shows the microhardness values shift from the margins, where the hardness values are higher than those in the core of the substrate, after measuring the microhardness of the coating layers of Y 2 O 3 -ZrO 2 doped Cr-Co aluminide diffusion coating at 1100 o C for 6hr with 0.5% Y 2 O 3 and ZrO 2 . This is because, during the coating process, NiAl is formed. Given the high degree of hardness of this compound, it is anticipated that the hardness of the coating layer will decrease as it approaches the substrate's center; however, this also depends on the alloying components of the base alloy. The findings show that the coating's hardness is increasing in certain areas and decreasing in others. This is a result of the formation of relatively brittle phases caused by the saturation of those regions with aluminum at high temperatures [21].

Hardness Measurement
In addition, Y 2 O 3 and ZrO 2 are very hard in comparison to alloying elements. In some areas of the IDZ, despite being close to the substrate, the hardness value is very high. This is because white precipitates containing aluminum, copper, and chromium have formed in these areas.

Oxidation Behavior
The step of restricting the total rate of reactive oxidation rate data is typically employed as the basis for a quantitative description of oxidation behavior, and oxidative kinetics studies offer useful information about the oxidation mechanism [18]. Weight gain was recorded to determine the kinetic mobility in dry air at temperatures (850,950 and 1050 o C) for up to 50hr in a 5hr cycle. Specific weight change data from the Inconel 625 alloy are drawn from the oxidation test as shown in Figure 7 as a function of time. Primary motility is swift, but over extended periods, the particular weight-change rate steadily declines. By looking at the fixed time growth rate or the value of n, which is obtained in the following equation, oxidation kinetics can be predicated [18]: where ∆W is the weight change, A is the surface area of the sample, K is the oxidation rate constant, t is the oxidation time and n is the growth rate time constant. Computer software was used to calculate the n-value for the temperatures (850, 950, and 1050 o C) based on the equation that fits the data the best. The exponential constant n characterizes the oxidation rate as follows: n=1, the oxidation rate is linear, if n=0.5, the oxidation is parabolic, and if n=0.33, the oxidation rate is cubic. The oxidation kinetic does not follow a straightforward parabolic trend when the value of n is more or less than 0.5, and this suggests a quicker or slower oxidation rate. In addition to the interpretation given above, the experimental setting and other factors, such as weight measurement and area, may have a significant impact on thick cases [18].
For the parabolic kinetics, the rate equation takes the form [18] : K now refers to the parabolic rate constant. A plot of weight change vs. square root of time gives a line, the slope is the parabolic rate constant in units of ( mg / Cm2)/h1/2). The Kp value is then squared to give Kp in units of (mg2/ Cm4)/h), as in the following expression [21] : It should be noted that the above-described process is also used to get the other values of n and Kp in this study, as shown in Figure 7 ( a-c).
The oxidation kinetic curves that were exposed to air for 50 hours at 1050 °C are shown in Figure 8. On the surface of the Ni-base superalloy, Cr 2 O 3 was created during the oxidation process at 1050°C, and with additional oxidation, CrO 3 was produced. Cr would become volatile at temperatures above 950°C, inducing Cr loss from the outer-oxide layer and revealing severe oxidation. Outstanding oxidation [22], resistance was present in the coated specimens. This showed that the protected alloy continued to deteriorate at 1050°C, and the Al 2 O 3 scale formed on the covered specimens' surfaces. At all levels of oxidation, the mass change curves of coating follow the same law [23].
The quick oxidation of the coatings caused by the high temperature over the first 25 hours causes the weight gains to considerably increase. After the first oxidation, dense oxide layers that are protective may develop on the surface, protecting the coatings from additional oxidation. As a result, the mass gains gradually continue. The mass increase at oxidation temperatures of 1050°C and 950°C is marginally higher than that of 850 over the first 15 hours. A few, tiny pores and cavities were seemingly visible behind the oxide film, indicating that the substrate had undergone oxidation. It was caused by intergranular internal oxidation, which was brought on by oxygen atoms moving between the coarse substrate grains. However, after being exposed to air at 1050°C for 50 hours, a protective α-Al 2 O 3 scale developed on the coated specimen as shown in Figure 8. The oxide scale was continuous and dense. The substrate was also homogeneous, thick, and pore-free. The outcomes demonstrated that the substrate had been adequately shielded in a high-temperature setting from additional oxidation. The diffusion rate of oxygen is drastically reduced when a dense oxide layer forms on the surface, but there is still a tiny amount of oxygen that could go into the coating and interact with the aluminum element. The alumina layer on the coating's surface is around 3±1µm, 4±1µm, and 6±1µm thickness, respectively.
Wagner's hypothesis states that if the less noble metal can diffuse sufficiently quickly into the coating/oxide contact, then the exclusive oxidation of this element can be accomplished [20]. Due to the fact that the Gibbs free energy of Al2O3 is much lower than that of oxides like NiO, Cr 2 O 3 , etc. and the Al content in the coatings is up to 50%, which is significantly higher than the minimum Al content required to sustain its exclusive oxidation, a dense and continuous Al 2 O 3 layer is formed on the surface.
As Y and Zr atoms could encourage the formation of the stable α-Al 2 O 3 phase and reduce internal stress in the oxide scale as shown in Figure 9, it can be deduced that adding Zr and Y to the Cr-Co modified aluminide coating might increase the oxidation-resistance [19]. Numerous studies reveal that the effects of Y 2 O 3 are generally accepted to be as follows [23,24]: (1) to promote the selective oxidation of chromium by lowering the critical content for the formation of a continuous chromium scale, and (2) to slow down scaling by switching the scaling mechanism from dominant outward chromium diffusion in the absence of RE to dominant inward oxygen diffusion [24].

Conclusion
The microstructure of the coating is composed of three layers outer layer, transition layer, and IDZ. The average coatings thickness was 132.37, 36.11, and 37.65µm for the outer layer, transition layer, and IDZ, respectively. XRD analysis of the coating system at 1100 o C for 6hr , it was found phases formed AlNi 3 , CoO, Al-Cr-Co and Cr 4 NiZr. The hardness value is highest at the top of the outer layer and decreases toward the sample core. The n and Kp value is increased with increased oxidation temperature. it was found that adding Y 2 O 3 and ZrO 2 to the Cr-Co-modified aluminide coating might increase the oxidation resistance.