INFLUENTIAL FACTORS ON THE TOUGHNESS OF NANOSTRUCTURED HARD METALS
Tamara Aleksandrov Fabijanic1, Ivan Jeren2, Vesna Puklavec2
1Faculty of Mechanical Engineering and Naval Architecture, Ivana Lučića 5, Zagreb
2Alfa Tim d.o.o., Čulinečka cesta 25, Zagreb
Corresponding author: tamara.aleksandrov@fsb.hr
ABSTRACT
The procedure of toughness determination on nanostructured hard metals is described in the paper. Toughness is determined with the use of Palmquist toughness
test by measuring the total length of cracks emanating from the four corners of a Vickers hardness indentation using the load of 294 N according to ISO 28079:2009.
The specimens of nanostructured hard metals were developed in the company Alfa tim d.o.o. by conventional powder metallurgy process varying the parameters related
to technological process of obtaining such as the sintering temperature and input variables such as percentage of Co and grain growth inhibitors. The measurements on
developed specimens were carried out in the Laboratory for Testing Mechanical Properties of Materials at the Faculty of Mechanical Engineering and Naval Architecture.
The results of the carried measurements and influence of certain factors were analysed. Certain conclusions about influential factors on the achieved toughness of
nanostructured hard metals have been brought.
Keywords: fracture toughness, Palmquist test, nanostructured hard metals
PALMQIUST TEST
The toughness value can be calculated using two methods:
Method 1 - Ratio of indent load to crack length , WG
WG
P

, N / mm
T
T  l1  l2  l3  l4
where :
WG Palmquist toughness
P – indentation force, N
T – total crack length, mm
Method 2 - Calculated value of toughness, WK
WK  A H WG , MN/m
3/2
where:
WK - Palmquist fracture toughness
A - is an empirical constant with a value of 0.0028,
H - is the hardness in N/mm at a load of 30 kgf
Figure: Vickers indentation with characteristic values
a)
b)
Figure: Microstructure of developed nanostructured hard metals on SEM
a) Microstructure of the sample 366-1, sintering temperature 1400°C, 10 000 X
b) Microstructure of the sample 376-1, sintering temperature 1420°C, 10 000 X
EXPERIMENTAL PROCEDURE
10
9,8
9,8
9,6
9,6
9,4
9,4
3/2
10
Wk, MN/m
Wk, MN/m
3/2
ANALYSIS OF THE RESULTS
9,2
9
8,8
9,2
9
8,8
8,6
8,6
8,4
8,4
8,2
8,2
8
1740
1760
1780
1800
1820
1840
1860
1880
8
1620
1900
1640
1660
DN 2-5 CRV9/366
WC-DN-2-5
DN 2-5 V-9/369
WC-DN-2-5
DN 2-5 CRV6/376
WC-DN-2-5
DN 2-5 CRV4/379
WC-DN-2-5
VC, Cr2C3
Grain
size,
nm
150
Co
content,
%
9
VC
150
9
VC, Cr2C3
150
6
VC, Cr2C3
150
4
10
9,8
9,8
9,6
9,6
9,4
9,4
3/2
10
Wk, MN/m
3/2
Grain grow
inhibitors
Wk, MN/m
Batch
Starting
powder
9,2
9
8,8
1740
1760
1780
1800
1820
9,2
9
8,8
8,6
8,6
8,4
8,4
8,2
8,2
8
1950
1960
1970
1980
1990
2000
2010
Vickers hardness, HV 30
Figure: Samples developed by
conventional powder metallurgy
process
1720
a)
b)
Figure: a) Palmquist fracture toughness versus Vickers hardness of the batch DN 2-5 CRV-9/366
b) Palmquist fracture toughness versus Vickers hardness of the batch DN 2-5 V-9/369
b)
Figure: Starting powders analyzed on FESEM
a) WC D-N 2-5 with addition of VC
b) WC D-N 2-5 with addition of VC i Cr3C2
Table: Batches characteristics
1700
Vickers hardness, HV 30
Vickers hardness, HV 30
a)
1680
2020
2030
2040
8
2060
2070
2080
2090
2100
2110
2120
2130
2140
2150
Vickers hardness, HV 30
a)
b)
Figure: a) Palmquist fracture toughness versus Vickers hardness of the batch DN 2-5 CRV-6/376
b) Palmquist fracture toughness versus Vickers hardness of the batch DN 2-5 CRV-4/379
CONCLUSION
On the basis of conducted research can be concluded that Palmquist toughness values do not change with the change of Vickers hardness obtained by sintering on
different temperatures. In another words sintering temperature doesn’t t influence on the achieved values of Palmquist fracture toughness of nanostructured hard metals.
Therefore can be confirmed that the Palmqiust fracture toughness does not decrease with increasing hardness values what is a case in conventional hard metals as
mentioned in already published literature. Measured values of Palmquist fracture toughness for the samples from the batches DN 2-5 CRV-9/366, DN 2-5 CRV-6/376
and DN 2-5 CRV-4/379 vary for approximately 2 % what can be attributed as a measuring error. Continuous small decrease of Palmquist fracture toughness is noted
only for the batch DN 2-5 V-9/369 where grains grow inhibitor is VC and differ from other bathes where grain grow inhibitor are VC and Cr3C2. Mentioned decrease is
very small and can be explained as a measuring error. Still, should not exclude the fact that grain grow inhibitors influence on the fracture toughness behaviour of
nanostructured hard metals. Further investigations are needed to be done to confirm or reject above mentioned fact.
Also, influence of cobalt contents on Palmquist fracture toughness is noted; the measured values of Palmquist fracture toughness decreases with decreasing cobalt
content.