A STUDY INTO THE MECHANICAL STRENGTH
OF THE WELDED JOINT BETWEEN A HIGH-TEMPERATURE
PIEZOCERAMIC AND A STRUCTURAL ALLOY
Yu. G. Zadorozhnyi
Translated from Novye Ogneupory, No. 12, pp. 43 – 46, December, 2003.
The effect of diffusion welding parameters on the mechanical strength of a welded joint is assessed. The role
of welding regimes in the strength improvement of a welded metal-piezoceramic structure is discussed.
Sensors based on piezoceramics (PC) have gained wide
acceptance in measurement technology. These are accelero-
meters, fluctuating pressure sensors, force cells, to name but
a few. They exhibit high operational stability and a wide
range of performance characteristics. There are piezoelectric
sensors that are capable of sustaining temperatures as high as
500°C. They use TV-2 and TV-3-type piezoceramics based
on bismuth titanate.
A specific feature of high-temperature PCs is that they
use a low-melting metal — bismuth whose concentration in
the composition may be as high as 70 wt.%. During the dif-
fusion welding of a PC and bismuth, the metal performs as a
dopant for the surface ceramic layer. This results in the oc
currence of a liquid phase at the contact of the welded mate
rials and may cause failure of the welded joint. Diffusion
welding of the PC-metal type can be carried out through a
chromium barrier layer (BL) 0.1 – 1.5 mm thick [1, 2].
Our goal in this work was to gain a deeper insight into
the PC-metal diffusion welding technology with a view to
specifying conditions under which a maximum strength of
the welded joint could be reached.
The fabrication technology of all-in-one metal-ceramic
structures involves a series of operations where each opera
tion exerts an influence on the mechanical strength of
the welded structure. For the joint TV-2 + Cr + Cu +
KhN67MVTYu-grade steel, a number of parameters can be
specified that affect the strength. These are polarization and
stabilization, the thickness of a copper compensation spacer,
PC surface roughness, welding temperature, compression
pressure, time, and chromium layer thickness.
To analyze technological implications in the mechani
cal strength of a welded structure, a single-factor design
was used. The process parameters were fixed such as to pro
vide maximum strength of the welded structure. To follow
the change in strength, one process parameter was varied
within a specified range. The measurements were carried out
on a welded structure in size identical with the actual dimen-
sions of a pulsating pressure sensor. The weld area diameter
was 5 mm. The height of the piezoceramic component was
3 – 6 mm. The tensile strength of the welded structure was
measured at a constant loading rate as recommended in .
During welding, the piezoceramic component is exposed
to compression pressure and to high temperature, which may
cause a change in the component’s stoichiometric composi
tion and lead to a depolarization. As was shown by measure
ments, an intense mass loss (over 1%) in the TV-2-type PC
accompanied by a change in stoichiometry was observed at
temperatures above 1213 ± 10 K. The TV-2 PC sustained
heating at 1153 K under a vacuum of 1.33 ´ 10
Pa and a
static compression pressure of up to 20 MPa for a period of
600 sec. These conditions are a marked improvement over
the conventional regimes of ceramic-to-metal diffusion
welding using a copper spacer [3 – 5]. Polarization and stabi
lization of the piezoceramic component according to a stan
dard polarization cycle  improved but slightly the me
chanical strength of welded structure. The increase in
strength did not exceed 15%. Further thermal stabilization to
a maximum operating temperature of the component (773 K)
caused a slight decrease in strength of the welded structure.
The overall increase in strength due to polarization and stabi
lization did not exceed 10%.
Mechanical grinding of the PC surface. Using ground
surfaces with a surface roughness parameter Rz = 0.4 – 0.6 mm
allows the strength of a welded structure to reach a maxi
mum. Increasing the surface roughness (Rz =1.5–2mm,
Refractories and Industrial Ceramics Vol. 45, No. 2, 2004
1083-4877/04/4502-0111 © 2004 Plenum Publishing Corporation
Kontakt Design Office, Chernigov, Ukraine.