Thermal Stability of Ni-P and Ni-Cu-P Amorphous Alloys N. Krasteva, V. Fotly, and S. Armyanov* Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria ABSTRACT The thermal stability of electroless Ni-X-P alloys is important for many applications in the electronics industry. This property of amorphous Ni-P and Ni-Cu-P alloys prepared using two different methods (melt-spinning technique and electroless deposition) is compared by differential scanning calorimetry. Analysis of the crystallization kinetics in both cases is carried out and its parameters are determined. Rapidly quenched alloys show higher thermal stability, while the total heats of crystallization are equal for both preparation methods. It is shown that the introduction of copper into electroless deposited Ni-P alloys exerts two favorable effects: an increase in crystallization starting temperature and a decrease in magnetization of the ferromagnetic phase precipitated after annealing. The thermal stability of electroless Ni-X-P alloys is im- portant for many applications in the electronic industry. An example for this is the thin-film memory disk sublayer, which must have paramagnetic behavior. The demand for higher coercivity media leads to an increase in tempera- ture, at which the magnetic film is sputtered onto the Ni-P layer. Since the amorphous state is metastable, there is a natural tendency for transformation into the more stable crystalline state, when the temperature is elevated. This transformation, as is shown, makes the Ni-P sublayer fer- romagnetic and the thin-film disk unusable. In this way the level of magnetization in the paramagnetic state is impor- tant for new generation thin-film disks. This is the reason for measuring it after annealing at 250 to 300~ for 1 to 3 h. ~Thus the knowledge of the mechanism of crystalliza- tion is necessary to prevent it or to regulate it within a predetermined range. Crystallization processes in Ni-P layers prepared by four different methods: electroless plating, 2-8 electrodeposi- tion, 9-~ melt-spinning technique, ~6-2~ and sputtering n are studied intensively using differential scanning calorimetry (DSC). The key temperatures Txp, T=, and Txr (Fig. 1) depend on the heating rate and actually denote temperature inter- vals. Here T= is the crystallization temperature, above * Electrochemical Society Active Member. &p tx Z mx (Txp,Txr) T x Txr Fig. 1. Differential scanning calorimetry curves of the crystalliza- 12 tion processes in amorphous Ni-P alloys shown schematically (a) hypoeutectic alloys, (b) eutectic alloys, and (c) hypereutectic alloys. 2864 which the sample is fully crystalline. Txp is related to the precipitation of Ni crystallites (precrystallization tempera- ture), involving a change of the amorphous matrix and an increase in its P content. At the recrystallization tempera- ture, T~, an off-stoichiometric (Ni-rich) Ni~P2 phase de- composes into final equilibrium phases: crystalline Ni and Ni3P. Complete crystallization, i.e., equilibrium state is reached only after annealing at elevated temperatures for several hours. The amorphous alloys with eutectic compo- sition may crystallize forming interstitial Ni-rich (off-stoi- chiometric) Ni3P phase. All this has been established by transmission electron microscopy (TEM) and x-ray diffrac- tometry accompanying DSC studies. In some cases meas- urements of magnetic properties, 1~ and hardness n also have been conducted. The crystallization processes of amorphous Ni-P involve intermediate phases (often more than one). There are some interesting, yet not completely understood, features of this transformation including the spinodal decomposition. ~~ In contrast to Ni, Ni3P is paramagnetic. For our consider- ations it is important that Ni crystallites can be obtained by different types of transformation: precrystallization, crys- tallization, and precipitation from the off-stoichiometric phases. Theoretically the eutectic alloy displays the great- est stability. It was proved experimentally, that electroless amorphous Ni-P alloys with composition in the range of 20 to 22 atomic percent (a/o) are the most stable. 7'17 The simul- taneous study by DSC of amorphous materials with the same composition, but obtained by different methods, may reveal the difference in the structure of these alloys in as- prepared state, and thus may contribute to our better un- derstanding of the transformation mechanism amor- phous --->crystalline state. It can also help to expand the applications area of electroless amorphous Ni-P alloys. Our aim here is to compare the thermal stability of Ni-P and Ni-Cu-P amorphous alloys obtained by rapid quench- 2.0 & 1.0 Scan rate r k/rain NI61 P19 AT% f. -- !/ I i I V i i 630 0.0 i i i i i i sso ~9o 670 71o "TEMPERATURE (K) Fig. 2. DSC curves of eutecfic Ni-P amorphous alloys: 1-MS Ni-P; 2-EL Ni-P. d. Electrochem. Soc., Vol. 141, No. 10, October 1994 9 The Electrochemical Society, Inc.