Page 23 - Demo
P. 23
Issue 143 August 2024 PCMI Journal 23
PCM of Difficult-to-Etch Metals and Alloys: Nickel- and Cobalt-Based Superalloys
Emeritus Professor David Allen, Cranfield University
Table 7. Nominal compositions of some nickel-based superalloys.
Alloy Ni
%
Cr
%
Mo
%
Co
%
Al
%
Ti
%
W
%
C
%
Fe
%
Hastelloy B
N10001
58-64 1.0 max 26-30 2.5 - - - 0.05 4-6
Hastelloy B-2
N10665
65-74 1.0 max 26-30 1 max - - - 0.02
max
2 max
Hastelloy C
-
Balance 14 15 2.5 - - 3 0.08 5
Hastelloy C-276
N10276
≈ 56 16 16 2.5 max - - 4 0.01 5
Hastelloy R-235
-
Balance 15.5 5 0 2 2.5 - 0.15 10
Hastelloy X
N06002
Balance 20.5-23 8-10 0.5-2.5 - - 0.2-1 0.05-
0.15
17-20
René 41
N07041
56.9 19 10 10 0.87 3.1 - 0.09 5
Udimet 500
N07500
53 17.5 4 16.5 2.9 3.9 - 0.05-
0.1
4
Udimet 700
N07700
Balance 15 5 15-20 4.4 3.4 - 0.05-
0.14
0
Waspaloy
N07001
55.1 19.5 4.3 13.5 1.5 3 - 0.08 2
Llewelyn Stearns, CA, USA [18] used a double-coating technique to improve the
adhesion of the maskant onto the superalloy surface. He coated the surface with
an epoxy resin, cured it and then dip-coated a photoresist on top of the epoxy.
After exposure and development of the resist, he then removed the epoxy
through the resist apertures with concentrated sulphuric acid. Using this
technique, an etchant comprising 32% hydrochloric acid (9 US gallons), 40°Bé
nitric acid (1 US gallon), 70% hydrofluoric acid (0.2 US gallon) and anhydrous
ferric chloride to obtain a specific gravity of 1.26, he was able to etch Hastelloy
B, Waspaloy, René 41, Inconel 617, Inconel 625 and the cobalt-based superalloy
Alloy 25 at rates of 6-13 μm / minute.
It should be noted that Stearns’ patent of 1981 has been superseded by the
invention of an epoxy photoresist that was not available at that time. SU-8 was
first patented by IBM in 1989. By using an epoxy photoresist, only one coating of
the metal is required. Examples of liquid and dry film epoxy photoresists that can
be purchased now include SU-8, the original liquid resist that can be applied by
dip-coating; SUEX, a Thick Dry Film Sheet (TDFS), 100 μm to over 1 mm thick and
