PREPARATION AND CHARACTERIZATION OF ELECTROLESS Ni COATED NANO ALUMINA POWDER UNDER DIFFERENT SENSITIZATION-ACTIVATION CONDITIONS

The development of electroless coatings on various substrates has gained much interest among researchers for the sake of improved properties. However, the coating on ceramic particles as a source of reinforcement is still a challenge for the researchers and requires a good comprehension of fundamentals since the coating thickness relies upon many parameters. Particularly the sensitization and activation conditions are more important for the creation of an ideal environment to draw metallic ions as a coating layer. Therefore, this paper examines the role of sensitization and activation conditions on the viability of nickel coating on the alumina particles of an average size of 50 nm. A comparison is made between two environments, namely individual and blended activations in the preparation of coated particles. Characterization studies are also presented to support the discussion.


Introduction
Composites are one of the key areas of research and grown in an exponential way as these materials possess lightweight and high strength in economic applications. Particularly, Metal Matrix composites (MMCs) have made a great impact in replacing the conventional monolithic materials. Alumina is the most commonly used material in modern MMCs as a source of reinforcement, given its desirable characteristics [1][2][3]. However, the preparation of a composite material needs special attention in achieving (i) low porosity (ii) uniform distribution (iii) wettability and (iv) matrixreinforcement interactions for the enhanced properties [4]. Some of the issues mentioned above were solved by changing the process parameters; however, the interactions play a significant role in deciding the performance of the composite. To promote enhanced interactions between matrix and reinforcement, coating the ceramic particulates with metal is found to be one of the prominent methods in recent times [4][5][6][7][8][9].
In general, a metallic coating can be done by using Electroless plating [5], Sol-Gel method [10], Heterogeneous Precipitation [11], and ball milling [12] etc., but Electroless plating has been identified as one of the effective techniques in view of simplicity and unique properties. Nickel and copper plating are the most commonly used electroless plating materials for industrial applications, while an electroless nickel coating gives particularly great hardness, fatigue strength, protection from wear and corrosion with a uniform covering over a wide scope of base materials [13].
The electroless plating process has two steps viz. surface preparation and bathing. This process consists of many parameters that directly affect the coating thickness [14][15][16][17][18]. Leon et al. [5] described the electroless nickel deposition process on micro-level SiC and Al2O3 particles and observed continuous nickel films on both the particles. Mehmet Uysal [6] discussed the influence of chemical quantities and powder concentration on the preparation core-shell Al2O3/Ni powders and further added that maintenance or control of these process parameters is essential for effective plating. Pang et al. [14] and Sameer et al. [19] emphasized the importance of sensitization and activation to attract the metallic ions on nano-ceramic particles. A two-step method of sensitization and activation is the most widely used methodology for surface preparation, while the one-step activation technique is becoming popular in the recent years as this process is quite simple with an economic advantage [20].
Therefore, this work confers to examine the effectiveness of coating achieved by both two-step and one-step activation methods on nano-sized Al2O3 powders prepared through standard nickel electroless plating procedures. The mechanism, experimental conditions, and the influence of these conditions on coating are elaborated in the later sections along with the characterization studies conducted.

Raw Materials
99% pure Al2O3 powders of average particle size 50 nm, supplied by M/s United Nano Tech Products Limited (UNTPL), Howrah, were used in this experiment.

Principle
The basic mechanism of electroless plating procedure involves the reduction of a metallic ion in an aqueous solution containing a reducing agent along with accelerators. This metal ion is attracted to the nucleation sites created on the surface of the substrate via the pre-treatment process [18]. Sensitization and activation procedures make the conditions favorable for plating. When the activated Al2O3 powders are added to the bath, the metallic nickel could be deposited with the following redox reactions. The detailed stoichiometric reactions are mentioned through Equations 1-3, while the mechanism for the plating process is shown in Figure 1.

Experimental Procedure
The chemicals and process parameters used in the pre-treatment process are mentioned in Table 1. Ultrasonic cleaning of particles was done after each step for better nucleation [21]. After pre-treatment, particles were dried and forwarded to the electroless nickel deposition bath. The chemicals used, their functional significance alongside their corresponding quantities were given in Table 2. The alkaline bath was considered in the present investigation as these produce low phosphorous content [20,21]. activation [14].

Effect of Sensitization Conditions
To study the impact of sensitization and activation conditions, two conditions we investigated in the present study. The first, after proper cleaning of the surface, the sample of Al2O3 powder underwent sensitization for SnCl2 and later activation for PdCl2 (Sample B). While the second condition, the single-step activation method, a combined solution of SnCl2 and PdCl2, was prepared for pre-treatment (Sample C). The samples were designated as given in Table 3.

Results and discussion
The density of all the samples was measured using the Archimedean principle and indicated in Figure 2. The density of Sample A is 3.90 g/cm 3, while the densities of Sample B and Sample C are 4.15 g/cm 3 and 4.09 g/cm 3, respectively. In both cases, Sample B and Sample C have increased density compared to Sample A. This increase in density may be attributed due to the addition of metallic Ni on the surface of Alumina ceramic [4,5,14,19].

Fig. 2. Densities of Samples A, B and C.
The XRD patterns of samples were shown in Figure 3. Figure 3(a) has the diffraction peaks of Al2O3 (Sample A) while Figure.  Furthermore, it has been reported in the literature that the wider peak gives preliminary information of more coating thickness with a small crystal size [14,19]. Therefore, Sample B seems to have more deposition of metallic Nickel around the alumina (Figure 3 (b)). Thus, the XRD analysis also indicates the presence of Ni and Al2O3 in the prepared powders along with preliminary results.  Figure 4 illustrates the FESEM images of nano-sized Al2O3 powders before and after electroless deposition with different sensitization and activation conditions. It has been observed that the surface of pure alumina (Sample A) was very plain (Figure 4(a)), while a coating has been observed on the particles was seen in Sample B and Sample C (Figure 4 (b) and (c)). From Figure 5, the traces of Ni, P, O, and Al were observed in the EDS analysis of Samples B and C as an indication of Ni-P coating. Both samples exhibited similar kinds of results in the characterization studies [ Figure 5 (a) and (b)]. The combination of Nickel -Phosphorous content is appeared because of the reducing agent used in the study [6,[22][23][24]. The studies are further carried towards coating thickness and crystallinity in assessing the quality of coated particles. The TEM images of powder of Samples A, B, and C were shown in Figure 6. The surface of pure Alumina particle, Sample A, was round ( Figure 6 (a)), while the surfaces of samples B and C have a clear indication of a coating. Sensitization and activation conditions helped the particles to attract Nickel; hence the coating was observed in both the samples B and Sample C. The two-step method of sensitization and activation provided a good platform for the Ni deposition over the nano Al2O3 surface and is confirmed in accordance with FESEM results along with EDS. The coating thickness was more uniform and varied between 2 nm to 31 nm of the particle in the case of Sample B (Figure 6 (b). The two-step method helps to create more nucleation sites as such, the sensitization and activation are done in separate baths. The pores on the surface are activated after the complete sensitization of the surface. The deposition process begins at random sites and keeps on developing to cover the full particle over time as a core shelllike structure [15]. The coating in Sample C does not have uniform coating like Sample B. For sample C, Metal island type distribution was observed as because of the single step activation method (Figure 6 (c)). M. Schlesinger reported that the single-step activation leads to low coating thickness over the particles [20], as confirmed by the present study. The quantity of Sn +2 was not sufficient to create more nucleation sites to produce a uniform coating. The SAD patterns for Sample B and C were shown in Figure 7. The structure of Ni coated Nano Al2O3powder was crystalline in the case of sample B (Figure 7 (a) ), and it is a mixture of orderly with the disorderly crystalline grains in Sample C when speculated from Selective diffraction image ( Figure 7 (b)). From the results, it is observed that the pre-treatment and the surface roughness significantly promoted the uniform deposition of Nickel on the alumina surface. The mode of individual activation step is to be superior in the formation of catalytic sites for the nucleation process, and a high surface roughness produced by full sensitization favoured a lateral growth mechanism of the Ni-P deposit and to form a core-shell structure.

Conclusions
From the present work, the following conclusions are drawn: • This paper highlighted the importance of proper sensitization and activation conditions in achieving the surface-modified ceramics for the sake of improved properties. • The color change and density values have given the preliminary indications of coating while the results of XRD, FESEM, and TEM are analyzed and presented to observe the effectiveness of coating on ultrafine ceramic particles. • The two-step method or individual method of sensitization and activation conditions lead to the preparation of core-shell type Ni coated nano-sized Al2O3 powders while one step method produced island type structure. • Palladium (Pd) nuclei on substrate surfaces created more catalytically active sites to initiate the electroless plating process in the two-step plating process.
In case of mixed PdCl2/SnCl2 catalyst solution, the active sites consisting of both Pd-Sn alloy had a lower number of catalytic sites that resulted in a less effective attraction of metallic Nickel. • Both the two-step and the single-step sensitization and activation methods produced nickel coating on the nano alumina particles. However, the twostep method appeared to be desirable for more coating thickness, while the one-step method is simple and fast, but with a lower coating thickness than the two-step method.