Journal of the Less-Common Metals, 51 (1977) 163 - 164 0 Elsevier Sequoia S. A., Lausanne - Printed in the Netherlands
Gold layers on ceramics
C. E. HOLCOMBE and W. B. SNYDER Nuclear Division, Union Carbide Corporation, Ridge, Tenn. 37830 (U.S.A.)
Oak Ridge Y-12 Plant, P.O. Box Y, Oak
(Received June 24,1976)
In the design of high temperature (-> 650 “C) capacitance transducers, a metal that is stable in an oxidizing environment must be bonded to a ceramic substrate with a volume resistivity (at temperature) of about lo6 10s R cm. Suitable ceramic substrates (199% dense) chosen for this work were alumina (AlsOs)*, machinable glass-ceramic (MGC)** and boron nitride (BN)+. Of the many traditional techniques [ 1, 21 for ceramic to metal bonding, a combination of three methods was used in sequence to form a ceramic-gold interfacial bond: (1) physical vapor deposition; (2) annealing for diffusion bonding; and (3) electroplating. The physical vapor deposition step allows reactive metals that are not readily ion-plated (Ni, Cr or Y) to be used as an interlayer in an attempt to form a graded reaction layer on annealing (reacting with both the Au and the ceramic). Thus a reactive metal was vapor deposited++ (to -1 - 2 pm thickness) onto the ceramic substrate and covered by an equivalent thickness of vapor-deposited Au; the coated substrate was then annealed at 900 “C for 1 - 5 h in an argon atmosphere; after cooling, the conductive coating was electroplated to a final thickness of 0.025 - 0.037 mm. Stability of the coatings was determined by heat treating in air at 650 “C for one week. Specimens were examined by optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA), in addition to visual inspection. Using the same application procedure a sample with an Au coating and no interlayer was prepared on each of the ceramic substrates for comparison. For AlsOs, Cr and Y interlayers were tested; both coatings were qualitatively superior to Au with no interlayer. However, under the conditions used only the Cr penetrated into both the Au and the AlsOa forming a graded layer (Fig. 1). The total reaction layer thickness was approximately 2 pm. *Coors Porcelain Company, AD-998, 99.8% AlgOa. **Corning Glass Works, Code 9658, machinable glass-ceramic (see U.S. Patent No. 3,801,295) containing approximately 58% phlogopite mica KMgaAlSiaOro (OH,F)g and borosilicate glass (- 34% SiOa, - 8% BaOa). +Union Carbide Corporation, Grade HBR, 97.5% BN. t+Using Model KSE-2 vacuum evaporator (Kinney Vacuum Division, Boston, Mass with a point evaporation source from a heated tungsten wire basket (vacuum - 5 x 10 -ti Torr).
Fig. 1. Taper section (which provides an effective magnification of 5 - 10 X ) optical photomicrograph of gold-coated alumina with a chromium interlayer after annealing and electroplating: area 1 is the Cr-A1203 reaction region; area 2 is the Cr-Au reaction region.
For MGC, Cr and Ni interlayers were tested; the latter peeled on annealing in air, whereas the former showed slight blistering. Neither interlayer material adhered as well as Au with no interlayer (physically bonded). For BN, both Cr and Mg interlayers were tested. Although the adherence appeared to improve somewhat with respect to Au alone, the samples with Cr and Mg interlayers were non-conducting after annealing. All Au coatings (with or without interlayers) on BN deteriorate on exposure to air (at room temperature or above), presumably owing to hydration of the residual B,Os used as a sintering aid in the preparation of the BN. In conclusion, it appears that this technique provides a graded reaction layer only for Cr interlayers on AlaOs. The AlaOs-Cr-Au bonding should provide a superior bond from the st~dpoint of adherence, thermal expansion grade and overall air stability.
References 1 C!. I. Helgesson, Ce~mic-to-Meal Bonding, Boston Technical Publishers, Cambridge, Mass., 1968. 2 H. E. Pattee, R. M. Evans and R. E. Monroe, NASA Spec. Publ. SP-5052, 1968.