Surface Hardening of Complex Molybdenum Alloy Parts by Gas ...

Nigg, Martinz and Lechleitner 17th Plansee Seminar 2009, Vol. 1 RM 55/1
Abstract
Surface Hardening of Complex Molybdenum Alloy Parts by Gas
Treatment
B. Nigg*, H.-P. Martinz* and A. Lechleitner*
* PLANSEE SE, Reutte, Austria
Molybdenum is the high end material for hot runner nozzles (HRNs) when they are used for injection
moulding of corrosive polymers. For better abrasion resistance in addition the outer but especially the
inner surfaces of the molybdenum nozzle are hardened by SHN ® ( = Super Hard Nitride), a gas nitriding
process. To access longer lifetimes of the hot runner nozzles within this work attempts have been made
to enlarge the hardness, to enlarge the thickness and to improve the oxidation resistance of the
hardened zone. Variations of the process parameters as well as additions of elements like carbon or
silicon resulted in reasonable improvements of surface properties.
Keywords
Molybdenum, TZM, SHN, gas nitriding, oxidation, hardness, pack siliconizing
Introduction
Molybdenum and its alloys (like TZM) are the high end materials for hot runner nozzles when they are
used for injection moulding of corrosive polymers. For better abrasion resistance in addition the outer but
especially the inner surfaces of the molybdenum (alloy) nozzle are hardened by SHN ® ( = Super Hard
Nitride), a gas nitriding process which has been derived from literature and internal experience [1 – 5].
As has been stated in the preceding publication of this group [6] the hardness profile and the oxidation
resistance of the surface zone should be improved in order to enlarge lifetime of the HRNs (Hot Runner
Nozzles) which are used in plastics injection moulding (PIM) to provide a reproducible connection
between feedstock and moulds. For that reason process parameters have been varied and the elements
carbon (hardness) and silicon (oxidation resistance; [7]) have been implemented into the hard layer.
Experimental
TZM samples were disc like for the parameter variations and the addition of methane to the ammonia
and tube shaped with various diameters for the experiments where silicon was added to the diffusion
layer. Table I shows the parameters of the experiments which were performed. The hardened samples
were evaluated by macroscopy, XRD ( = X-Ray Diffraction) and cross sectioning with subsequent LOM

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( = Light Optical Microscopy), SEM-EDX ( = Scanning Electron Microscopy – Energy Dispersive X-Ray
Analysis) and hardness measurements by a Nanoindenter.
Results and discussion
The standard SHN process on TZM leads to the formation of a zone which mainly consists of Mo2N [6].
Table I and fig. 1 show that the substrate below this zone is also hardened slightly. However in a depth
of 40 µm below the interphase the hardness is approximately as low as in the bulk substrate material. In
fig. 2 light optical micrographs of two cross sections can be seen: on the left hand side that of a standard
SHN sample, on the right hand side that of an high temperature (HT) SHN sample.
Table I: Process parameters and results of the various hardening processes
process Gas / Pack Temperature mass gain thickness of HV* in HV* - 10µm HV* - 40µm HV* - 100µm
denomination composition in °C in mg / cm 2
standard SHN
zone in µm hardened zone deep in base deep in base deep in base
TZM
HT SHN
NH3 1000 1,05 16 16 6 3,5 3
TZM
Carbo SHN
NH3 1200 1,55 12 17 8 7,5 7
TZM
Silico SHN
NH3 + CH 4 1100 0,70 11,5 17 7 5 3
Mo Si + NaF + H2 1100 10,00 30 14 2,9 2,9 2,7
NH3 1000
1800
1600
1400
1200
1000
800
600
400
Hardness
(HV 0,01)
in kg/(mm) 2
* all HV ( = Vickers Hardness) values in GPa
Hardness vs. Depth - SHN
-15 -10
200
-5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Depth (µm) from the interface
Fig. 1: Hardness profile for two TZM samples hardened with the standard SHN process
Hardness sample 1
Hardness sample 2

Nigg, Martinz and Lechleitner 17th Plansee Seminar 2009, Vol. 1 RM 55/3
Fig. 2: Light optical micrograph of cross sections of a standard SHN (left) and a HT-SHN (right) TZM sample
The HT (1200°C) SHN process leads to a less homogeneous diffusion zone with a lower mean
thickness. However the hardness profile is different (see table I and fig. 3): starting from a slightly higher
hardness of the diffusion zone even in a of depth 100 µm from the interface the TZM exhibits a hardness
of 700 kg/(mm) 2 , which is more than double compared to the bulk value (300 at approx. 200 µm depth).
1800
1600
1400
1200
1000
800
600
400
Hardness
(HV 0,01)
in kg/(mm) 2
Hardness vs. Depth HT - SHN
200
-25 -15 -5 5 15 25 35 45 55 65 75 85 95 105 125 145 165 185 205 225 245 265 285 305 325 345
Depth (µm) from the interface
Fig. 3: Hardness profile for two TZM samples hardened with the HT SHN process
Hardness sample 1
Hardness sample 3
This is a very promising result. Less successful was the carbo nitridation at 1100°C. Although the
diffusion layer which was formed is rather uniform (fig. 5, left) the hardness profiles look very different
depending on the location where they were measured (fig. 4). As the carbo nitrided samples showed
locally blackened areas soot formation from the gas phase can be assumed. This can probably be
suppressed by lowering the methane content in the gas mixture so that more uniform hardening
conditions can be established in future. XRD measurements revealed that Mo2C, MoC, MoN and locally
Mo2N contribute to the composition of the hard zone. This mixture is slightly less hard than the hard

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zones formed by ammonia exposure (Mo2N, much less MoN). The right hand sided light optical
micrograph of fig. 5 was prepared from a pack siliconized Mo sample which was subsequently submitted
to a standard SHN process. The horizontal thick layer on top consists mainly of MoSi2, the greyish
uneven phase on top is most probably Si3N4 (XRD – results). The vertical zone was formed during the
SHN process on the pure cutted Mo surface: it consists mainly of Mo2N.
1800
1600
1400
1200
1000
800
600
400
Hardness
(HV 0,01)
in kg/(mm) 2
Hardness vs. Depth - Carbo-SHN
-15
200
-5 5 15 25 35 45 55 65 75 85 95 105 115 125 135 145 155 165 175 185 195 205
Depth (µm) from interface
Fig. 4: Hardness profile for two TZM samples hardened with the HT SHN process
Hardness 3 long. a
Hardness 3 long. b
Hardness 3 transv.
Fig. 5: LO micrograph of the cross section of a carbo nitride TZM (left) and of a silico nitrided Mo (right) sample

Nigg, Martinz and Lechleitner 17th Plansee Seminar 2009, Vol. 1 RM 55/5
Fig. 6 shows the hardness profile through the silicide zone down to the Mo substrate. There is a thin
layer in between, most probably Mo5Si3 (EDX). As can be seen also from table I the hardness has a
steep gradient into the molybdenum substrate. The layer hardness is lower compared to the nitrided or
carbo nitrided substrates. This may – to a small extent – be caused by the softer base material Mo
instead of TZM. From literature [7] it is known that MoSi2 should be softer than Mo nitrides or Mo
carbides, but much more oxidation resistant. The pack siliconizing of the samples was performed in a
way that the inner volume was not filled with pack mixture in case of the long and thin tubes (fig. 7, left).
Nevertheless the inner cylindrical surface (diameter 2.4 x length 23.2 mm 3 ) was siliconized completely by
the evaporation of silicon fluoride which is formed from NaF and Si. The thickness of the silicide layer
inside the long tube of fig. 7 lies between 20 and 50 µm (fig. 8). This result is important because the real
parts which eventually have to be hardened in that way – the HRNs (Hot Runner Nozzles; fig. 7, right) –
also have thin channels. More work must be done especially to control the thickness distribution and
roughness of the layers, furthermore to check the oxidation and abrasion resistance.
1600
1400
1200
1000
800
600
400
200
0
Hardness
(HV 15mN)
in kg/(mm) 2
-60 -40 -20 0 20 40 60 80 100 120
Fig. 6: Nanoindentation Hardness profile for a sample hardened with the silico SHN process
Summary and outlook
Hardness vs. Depth - Silico SHN
Depth [µm] from the interface
Up to now for the plastic injection moulding of very corrosive and abrasive polymer melts TZM
or SHN treated TZM HRNs are used. For further improvement of the standard SHN diffusion
layer various measures were performed and evaluated: Higher process temperatures for SHN
lead to a flater hardness profile, addition of hydrocarbon to the ammonia and a higher process
temperature show a comparable, but significantly weaker effect and – finally – silico nitriding (in
two steps) causes a modified silicide layer which has to be investigated intensely and optimized
in future.

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Fig. 7: Macros of silico SHN treated Mo tubes (left) and of a variety of TZM HRNs for PIM application
Fig. 8: LO micrograph of the cross sections of a silico nitrided Mo tube: longitudinal (left), transversal (right)
References
1. H.-P. Martinz and K. Prandini, Int. J. of Refractory & Hard Materials 12, pp.179 – 186 (1994)
2. I. Jauberteau., J.L. Jauberteau, M. Cahoreau and J. Aubreton, Appl. Phys. 38, pp. 3654 – 3663
(2005)
3. Y. M. Lakhtin and Y. D. Kogan, Metal Science and Heat Treatment 10 / 1, pp. 24 – 28 (1968)
4. T. Nakajima and T. Shirasaki, J. Electrochem. Soc. 144 /6, pp. 2096 – 2100 (1997)
5. W. D. Sproul, Contract No. NAS 8-37686 – Final Report for NASA Marshall Space Flight Center
(1993)
6. H.-P. Martinz and B. Nigg, Proceedings 20 th International Conference on Surface Modification
Technologies, Vienna, pp. 100 – 106, ASM (2006)
7. D. Manomaisupat, D.S. Wilkinson and A. Petric, Journal of Materials Science, Volume 33, Number
9, pp. 2319-2330 (1998)