Physical and mechanical impact of Y2O3 on (Cu-7Ag) electrical contact alloy

In this study, (Cu-7Ag) alloy was synthesized through powder metallurgy rout and explored the effect of adding yttrium oxide (Y2O3) with three different volume fractions 2, 4, and 6 vol.%. The effects were recorded on the electrical conductivity, the hardness, and the wear rate of the prepared samples. The base samples were sintered at 850 °C for 4 hours, while the composite samples were sintered with two holding times: at 550 °C for 2 hours and at 850°C for 4 hours. All of the sintered samples were homogenized at 777 °C for an hour followed by aging at 400 °C for 8 hours. The base alloy (Cu-7Ag alloy) showed higher conductivity but poor hardness and poor wear resistance. Best balance between the improvement of mechanical properties and suitable electrical conductivity had been obtained with Cu-7Ag+4vol.% Y2O3 composite sample.


Introduction
The high conductivity and mechanical strength of Cu-Ag alloys make them attractive candidates for electrical interconnections.As these alloys have been optimized for high strength and conductivity with a composition of Cu-7Ag, it has been reported that a metastable phase, commonly denoted as (α), appears during ageing treatments [1].It is known that the face center cubic (Ag) phase can precipitate out of the FCC Cu matrix [2].In general, copper and its alloys especially Cu-Ag alloys have a high electrical conductivity but a relatively low wear resistance.Some investigators have devoted themselves to improving these materials for their application in mechanical properties.Increasing the strength of the copper and its alloys involves many procedures.Shizuya and Konno in [1] studied the age hardening and precipitation behaviors of the (Cu-Ag) alloys.Rosochowski and his coworkers in [3] used the severe deformation for grain refinement and enhancement of the properties.Felicia et.al in [4] studied silver's impact on cast copper alloys' hardness, strength, and electrical conductivity.Robinson et.al in [5] studied the effect of silver addition in copper-silver alloys fabricated by laser powder bed fusion in situ alloying.Alaa H. Jaafar and Haydar Al-Ethari in [6] investigated the effects of adding graphite to the copper used for electrical contact applications.Osama Abdel Bari et al in [7] investigated the effects of adding Al2O3 particles to the copper used for electrical contact applications.Mu et al in [8] studied the effect of adding yttrium on the properties of copper-based contact materials.
As the electrical contact applications need materials with a high performance, copper alloys with a high conductivity and suitable wear resistance are much needed to satisfy the demand for such products.Therefore, the aim of this study is to enhance the hardness and the wear resistance while maintaining a good and suitable electrical conductivity by adding particles of Yttrium oxide (Y2O3) to Cu-7wt%Ag alloy with three different volume fractions: (2 ,4, 6 vol.%) and obtaining a combination of the best required properties in the composite.

Materials used in the study
Powders of Copper (Cu), Silver (Ag), and Yttrium oxide (Y2O3) of 99.9% purity were used as raw materials to prepare the alloy and the composite samples of the present study.The particle size of the powders was analyzed via (the better size 2000, laser particles size analyzer).Table (1) shows the particle size and the supplier of each powder.

Samples preparation
Two types of samples were prepared.The first sample is the base (Cu +7wt%Ag) alloy, while the second represents the alloy reinforced with 2, 4 and 6 vol.% of Y2O3.The preparation was started by mixing the powders due to the required percentage.Four hours of mixing were considered for the basic alloy (Cu-7Ag) while a mixing for 6 hours was used for the composite samples.Electro rolling mixer, type (STGQM-1⁄5-2) was used for mixing the powders.Alcohol with 10wt% was used during the mixing process.Cylindrical samples of 12mm in diameter and 15 mm in height were prepared in a CT340-CT440 style electric-hydraulic press utilizing a double-action steel die.For all of the samples, a compacting pressure of 825MPa was used.This value of the pressure was determined according to the highest green density as shown in Fig. (1).The sintered compacts were heat treated under argon gas via a furnace type (MIT-GSL1600X).There was an hour of homogenization at 777 °C, followed by 8 hours of aging at 400 °C [9].The samples then underwent microstructure observation, XRD analysis, hardness test, electrical conductivity measurements, and wear resistance tests.

Microstructure
Sintered, homogenized and aged specimens with 12 mm diameter and 13 mm height were ground by using SiC paper grits as (400, 600, 800, 1000, 1500, 2000, 3000, and 4000), polished by using diamond paste, and etched in FeCl3•6H2O solution at room temperature.After etching, distilled water and an electric dryer were used to wash and to dry the samples.The microstructure of the samples was photographed with an optical microscope.A uniform mixture of two crystalline solids Cu and Ag formed a substitutional solid solution that share a common crystal lattice (FCC) according to Hume-Rothery rules [10].When copper and silver form a solid solution, the copper atoms dissolve in the silver lattice to form a solid solution.The alloy consists of α-and β-phase, where, α is a solid solution of Ag in Cu matrix and β is a solid solution of Cu in Ag matrix.Clusters or colonies (α), small, bright or light regions, appeared in optical micrographs for the base alloy (Cu-7Ag) represents Ag-rich areas exist within matrix Cu-rich areas in dark color; this is the same as stated in [4].These colonies Ag-rich phases dissolved into the matrix after the homogenization treatment at 777 ℃, as shown in  As noticed in Fig ( 4a) the microstructure of the sintered (Cu-7Ag+4vol.%Y2O3), no solid solution was formed between the base alloy and the added ceramic oxide, so the particles of (Y2O3) appeared clearly distinct from the color of the matrix and were distributed more uniform after the homogenization treatment and aging.

SEM test
After sintering, the specimens were analysed using scanning electron microscopy (SEM) to observe the microstructure and identify the difference among the samples.Figure (5) shows the SEM data for the base alloy, which indicates a contrasting specific pattern for distribution of Ag light colour particles in Cu dark matrix and pore sites.In addition, Fig. (5b, 5c, and 5d) show (Cu-7Ag) alloy reinforced by Y2O3 a white hard particles of small size particles dispersed the matrix, shape, and distribution can be observed.A significant change in the microstructure of the alloy can be noticed with increasing of Y2O3.Furthermore, the SEM images revealed that the Y2O3 particles had a uniform distribution, indicating that the powder metallurgy method used was effective in achieving uniform properties.It shows the distributed Y2O3 particles.The Y2O3 particles act as barriers to dislocation movement, resulting in an increase in the strength and hardness of the alloys.More amount of Y2O3 causing lattice distortion, the stress field is generated by the lattice distortion and the surrounding elastic stress field interact.The interaction between stress fields hinders the movement of dislocations so that the yield stress of the alloy increases sharply, and the material is strengthened [11].

XRD test
The tests were performed via the XRD generator type (XRD-6000) with Cu target of 40 Kv. and 30 mA, scanning speed 6deg/min, and a scanning of (35ᵒ-85ᵒ).Figure (6) represents the X-ray patterns for the Cu-7Ag base alloy and Cu-7Ag+6vol.%Y2O3 after the heat treatments, where the peaks match with the standard reference code (96-150-9080) for the silvercopper compound (0.04/3.96) and the reference code (00-005-0574) for the Y2O3.2).These phases are formed through heating the samples to a certain temperature and remaining for a holding time during homogenization followed by aging.These particular phases are known for their remarkable strength and hardness, and they significantly influence the properties, particularly in terms of the strength, the wear resistance, and the hardness.The other samples (cu-7Ag+2vol.%Y2O3 and (cu-7Ag+4vol.%Y2O3) have the same result as (cu-7Ag+6vol% Y2O3 (showed in Fig. (6b) and table (2).Table (2) shows the increasing in the hardness of the base alloy and the composites in parallel with the steps of the heating treatments.Homogenization and then quenching in ice water caused the formation of supersaturated solid solution, which is responsible about the increase of the hardness.After 8 hours aging there was a further increase in the hardness due to the suppress of the discontinuous precipitation reaction in the alloy with low Ag contents or more primarily Cu dendrites [8].Adding Y2O3 to the base alloy (Cu-7Ag) causes the hardness of the composite to increase.The increase depends upon the added volume percentage of the ceramic particles.These added particles have high hardness.It bears part of the applied load, therefore the hardness and the strength of the material increases, so that samples have a long lifetime and a great-applied load needed to deform it.Y2O3 was incorporated into the copper alloy as a dispersion-strengthening phase due to its properties and high chemical stability [8,12].

Electrical conductivity
Figure (7) shows that electrical conductivity of (Cu-7Ag) base alloy increase with the heat treatments due to the decrease in conductive electron scattering [9].Hard particles of Y2O3 act as barriers prevent the flow of free electrons and thus reduces electrical conductivity.Each of homogenization and aging reduce the segregation in samples, make the concentration more homogenous and the uniformly distribution of the Y2O3 particles does not significantly reduces the electrical conductivity values of the reinforced samples, which is increasing in parallel with the steps of the heating treatments.The values were measured via Resistance measuring device type (Applent AT512 High Precision Resistance ohmmeter).

Wear test
The concept of a pin on a disk was used to analyze wear from dry sliding.ASTM G 99-04 [13] details the testing procedure.Test specimens for the dry sliding wear test were 12mm in diameter and 10mm in height.The specimens were weighed on an electric balance that was accurate to within 0.0001.A wear tester (model MT-4003, revision 10.0) was used for the evaluation.The test specimen was mounted on a pin and rotated against steel disc with a 3mm radius.The load applied to the disc was 15 Newtons, and its rotational speed was set at 250 revolutions per minute.After 5, 10, 15, 20, and 25 minutes, the samples were weighed to calculate the dry sliding wear rate using Eq. 1 [14].
Wear rate (r = ∆/)   9c and 9d) shows a shift from adhesion wear to abrasive wear, which is represented by sharp lines that appear clearly on the tested surface [15].

Grooves a b
Wear area

Conclusion
The critical results obtained from this work can be summarized as follow: - Heat treatments processes has an active role in improvement the hardness of the Cu-7Ag alloy.
Homogenization at 777°C for one-hour followed by aging for 8 hours at 400 °C improved the hardness of the sintered compact of this by 86%.Reinforcing the alloy with 2, 4, and 6 vol.%Y2O3 recorded a further increase of 3, 8, and 12% respectively.
 Adding the ceramic oxide (Y2O3) has an effective role in improving the wear resistance of the Cu-7Ag alloy as this oxide reduces the wear rate by 54%.
 The electrical conductivity gradually improved during the heat treatments because these treatments reduce the segregation and increased homogenization.On the other hand, adding the ceramic oxide reduces the electrical conductivity.A maximum reduction of 27% was recorded due to adding 6 vol.%Y2O3, but the minimum recorded reduction was 10% due to adding 4 vol.%Y2O3.
 Best balance between the improvement of mechanical properties and suitable electrical conductivity had been obtained with Cu-7Ag+4 vol.%Y2O3 composite sample.

Figure 1 .
Figure 1.Effect of the compacting pressure on the green density The sintering of the green compact was carried out in a furnace type (MIT-GSL1600X) in an argon atmosphere.It was performed according to the program described in Fig. (2).The samples were left to cool inside the furnace.

Figure 7 .
Figure 7. Electrical Conductivity Vs different heat treatments

Figure 8 .
Figure 8. Wear rate for base alloy and composite specimensBase alloy sample shows the higher wear rate, while (Cu-7Ag+6vol.%Y2O3) records the lowest wear rate.This is attributed to the increase in the hardness due to the added ceramic particles.

Figure
Figure (9a)  shows adhesive wear that occurs when surfaces slide against each other, and the pressure between the contacting asperities is high enough to cause local plastic deformation and adhesion.When in motion, a small number of asperities stick together, and their size grows over time.Due to the ductility of the base alloy sample, eventually the junctions rupture at their weakest point, resulting in metal transfer from one surface to the other.The deformation of the surface became less pronounced by adding the ceramic oxide (Y2O3).Noticeable improvement of the surface begins in the composite sample having (2vol.%Y2O3) as shown in Figure(9b).Figures(9c and 9d) shows a shift from adhesion wear to abrasive wear, which is represented by sharp lines that appear clearly on the tested surface[15].

Table 1 .
The particle size and the supplier of the used powders

Table 2 .
The table below records the details obtained by XRD.

Table 3 .
Table (2) shows the results of the hardness tests.Hardness values after treatments for each sample The 14th Asia Conference on Mechanical and Aerospace Engineering Journal of Physics: Conference Series 2746 (2024) 012050 IOP Publishing doi:10.1088/1742-6596/2746/1/012050