Preparation and Characterization of Ti/Ni Core-Shell Structural Powders

Jing Maoxiang, Cai Yixiang

Department of Powder Metallurgy for Guangzhou Research lnstitute of Non-ferrous Metals ,Guangzhou, 510650 ,China

50Ti/50Ni thermal-spray coating possesses excellent properties of cavitation erosion resistance, which can greatly increase 3-4 times of lifetime

for marine propellers. Some studies have proved that preparing equivalent molar ratio Ti/Ni core-shell structural composite powders is crucial to the properties of thermal spraying coating. The Ti/Ni core-shell structural composite powders were easily prepared by combing heterogeneous

precipitation method and thermal-reduction process in this paper. The experiment results show that the prepared Ti/Ni powders have core-shell

structure, the Ti powders are evenly and densely enwrapped by Ni phase, and no free Ni phase or Ti phase is found, the composition ratio of Ni

to Ti is equal to I. The composite powders have narrow particle size distribution of 40-60 µm.good dispersity and fluidity,which can satisfy the

needs of thermal-spray coating for cavitation erosion resistance.

Keywords: Ti/ Ni composite powder ,core-shell structure ,hetero1<e11eous precipitatio11 .then11al-spray

I. Introduction

Cavitation erosion is one special erosion-corrosion, which widely occurs on the high speed marine propel­lers, turbine blades, supersonic speed foodstuff mixing arms, etc. 50Ti/50Ni thermal-spray coating possesses excellent properties of cavitation erosion resistance, which can greatly increase 3 - 4 times of lifetime for marine propellersI>. The traditional preparation method is that utilizing TiNi alloy powders as raw material and adopting low pressure plasma spray process, which can obtain high densely and high binding strength Ti-Ni al­loy coating. However, this method has many disadvan­tages such as high cost, low production efficiency, and easily limited by the dimensions and shapes of the workpiece1. 2>. So choosing other methods to prepare 50Ti/50Ni coating is becoming the aim of many re­searchers. K. S. Zhou, et al found that an excellent 50Ti/50Ni alloy coating was prepared via using 50Ti/ 50Ni core-shell structural powders as raw material and adopting low temperature high velocity oxygen-air fuel spray ( L T-HVOF) followed by laser modification treatment3>. Nevertheless, the preparation of 50Ti/ 50Ni core-shell structural powders is crucial to the coating properties. The traditional methods include ball milling, electroless plating, electroplating and high pressure hydrogen reduction method, etc. Thereinto, the Ti/Ni powder prepared by ball-milling method is an physical mixed power in fact, which always results in high oxygen content, inhomgeneous composition and long alloying time, etc. By other methods the core-shell structural Ti/Ni powders can be achieved and avoid the above shortcomings of ball milling method, but it is dif­ficult to achieve equivalent molar ratio Ti/Ni composite powders, low production efficiency and free Ni or Ti powder always occurred are also the main holdbacks. For the above-mentioned reasons, the equal molar ratio Ti/Ni core-shell Structural composite powders pre­pared by heterogeneous precipitation method were studied in this article.

2. Experimental Procedure

2.1 Preparation of Ti/Ni Core-Shell Structural Powders

The Ti powders with particle size of about 25 µm were from Titanium Corporation Limited of China. Other chemical reagents used were commercially avail­able from Shanghai Chemical Reagent Corporation Limited of China.

The preparation process of Ni coated Ti powders by the heterogeneous precipitation is described in Fig­ure 1. The preparation process consists of two steps. First, the precursor with Ni~01 • nH2 0 coated Ti powder was prepared by feeding Ni sulfate solution ( O. 5M NiS01 ) and oxalic acid solution ( O. 7M, H2~01 ) at the same velocity of 10 ml/min by two charge pumps into a reactor, in which Ti micro-pow­ders ( 50g/L) were suspended in distilled water with vigorous mechanically stirring at 50°C. The total molar quantity of NiS01 is 1. 1 times to Ti, and the quantity of H2~01 is 1. 4 times to NiS01 • After charging fin­ished, the precipitates were filtered and washed three times with distilled water, followed by drying at 60-800C for 12 hours to form the precursor ready for the subsequent hydrogen reduction.

Sulfate solution0.5M Oxalic acid 0.7M

stirring al so·c

Ti suspending solution

Mixing-rca ion, filtering,

washing. drying

Coated precursors

Reduction at 500"C hy hydrogen

Ti/Ni core-shell powder

Figure I. The flow chart of preparing Ni­

coated Ti powders

12. Marine Applications • 2255 •

Second, i coa ted Ti core-shell structural powders were obtained by the thermal reduction of the as-pre­pared precursor at 500°C for 2 hours under hydrogen

atmosphere.

2. 2 Characterization The morphology and structure of the as-prepared

precursor and resultant Ni coa ted Ti powders were characterized by scanning electron microscope ( SEM, JSM-2800 LV , J eol company ) and optica l microscope ( 9XB-PC, Shanghai Optica l Instrument Plant) . The crystalline phase and molar rat io of the Ti/ Ni powders were checked by X- ray diffraction spectrometer ( XRD, D/ max-rA,Rigaku ) with Cu Ka1 radiation and chemi­cal element analysis instrument ( KOC-6 , Nanjing Kaidi Analysis Instrument Co. ) respecti vely.

3. Results and Discussion

3. 1 Preparation of NiCiO, • nH2 0 Coated Ti Precursors The coating of iCz O, • nH2 0 on Ti micropow­

ders is actually a heterogeneous precipitation process. The precipitation consists of a eries of steps : mi­cromixing, reaction , heterogeneous nuclea tion and growth. According to the heterogeneous precipitation theor/> , there are a few factors needing cons ideration. First, the Ti micropowders are di spersed uniformly and suspended in reaction solutions to promote newly nu­cleated particles precipi ta ted on the their surfaces. (2) The nucleation and particle growth rates are tailored by micromixing reactants and controlling other processing factors such as charging velocity and temperature to keep a concentra tion supersa turation less than the crit­ical value resulting in a homogeneous nuclea tion so as to a void the emergence of dissociating preci pi ta ti on. ( 3) The concentration of the Ti micropowders has an opti­mized value which provides enough heterogeneous nu­cleation sites.

Based on the above considera tions , it was found that uniform and compact coatings on Ti powders were obtained as shown in Figure 2 with the optimized pre­cipitation conditions : 50g/ L of Ti micro-powders , l OmL/ min of charging velocity for 0. 5M nickel sulfa te solution and the quantity is 1. 1 times to Ti powder, lOml/ min of charging velocity for oxalic acid solution and the quantity is 1. 4 times to Ni ion, cons tant tem­perature a t 50°C , but no need to adjust pH va lue. From Figure 2, it can be also found that on the whole no dissoci­ating Ni(z O, • nH2 0 precipitation or uncoated Ti powder occurs in the precursors , and the dispersi ty of the compos­ite precursors is very good , which can be favorable to spray in control of composition and Ti oxidation.

3. 2 Preparation and Characterization of Ti/Ni Core­shell Powders In order to clearly observe the formation proce-

Figure 2. Pictures of NiC,O, • nH2 0 coa ted T i precursors

taken by optica l microscope

dure of the T i/Ni core-shell structure , 20 % , 50 % and 100 % Ni coated precursors were gathered and reduced at 500°C for 2hours , inwhich 20% ' 50% or 100% means the ratio of the charged quantity of i sulfate to the total quantity. The SEM of different ratio T i/ Ni powders were shown in Figure 3. Figure 3 (a) repre­sents the 20 % i/ Ti powder, which shows that Ni par­ticles were unevenly loca ted on the surface of Ti pow­der, and some surface area of Ti core were still uncov­ered. With the increase of Ni content, the surface of Ti powders were gradually covered by even and compact Ni particles as shown in Figure 3( b) and Figure 3(c), no free Ni phase or uncovered Ti phase was found. So the Ti/ Ni core-shell structure was finally formed. In addition, as shown in Figure 2 and Figure 3 ( c) the Ti/ Ni composite powders have narrow particle size di stri­bution of 40~ 60µm, which results in a good fluidity.

Figure 4 shows the XRD pattern of l OO% Ni coa­ted Ti powders derived from hydrogen reduction at 500°C of final precursors , it can be found that the main crys talline phases include Ni and Ti phase , a little im­purity phase such as Ti i alloy also exisits. According to the result, it can be inferred that iCz O, • nH20 shell of the precursor was completely reduced by hy­drogen under the designed conditions , meanwhile the Ti core was well protected by i shell to avoid oxidation, the appearance of TiNi alloy phase is probably resulted from the trace interfacial reaction between Ni shell and Ti core.

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• 2256 • Proceedings of the 12'h World Conference on Titanium

Figure 3. SEM of different molar ratio Ti / Ni powders Ca)20%, (b)50 % , Cc)lOO .%

Ni Ti

Ti

30 40

Ni

50 20/(° )

60 70

Figure 4. XRD patem of Ti/ Ni core-shell structural powders

The composition content of T i/Ni powders was analysed by chemical element analysis method as shown in Table 1. The main elements are Ti and Ni, and the atomic ratio is nearly equal to 1. However trace amout of C and 0 element were also detected, which is probably from the decomposition of oxalate.

Table 1. Element atomic content of the resultant Ti/ Ni powders

Element

Ti

Ni

0

c

4. Conclusions

Atomic content Cat%)

49. 98

49. 96

0.01

0.05

The heterogeneous precipitation process was suc­cessfully used for the preparation of Ti/Ni composite powders. The T i/Ni powders derived from optimized conditions have core-shell structure and equivalent mo­lar ratio, no free Ni phase or uncovered Ti phase is found. These composite powders have narrow particle size distribution of 40~60 um, good dispersity and flu­idity, which can satisfy the requirements of LT-HVOF spraying for cavitation erosion resistance.

Acknowledgements The authors wish to thank Guangzhou Research

Institute of Non-ferrous Metals for funding and sup­port.

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